CN115036454A - Safe and stable's activation negative pole - Google Patents

Abstract

The invention provides a safe and stable activated cathode, which is formed by introducing Li in a lithium-rich phase into an amorphous LiSi alloy cathode ₁₅ Si ₄ 、LiC ₆ The lithium conducting network in a three-dimensional texture shape is woven, 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 alloy and hard carbon ratio parameters, and successfully obtains the all-solid-state battery with high load, large current and long cycle capability.

Description

Safe and stable's activation negative pole

Technical Field

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

Background

At present, the solid-state full battery has great difficulty and challenge to realize the load and the area capacity of a commercial lithium ion battery and realize stable long circulation, and the high load (the area capacity is not less than 1 mAh/cm) reported in the literature ² ) The negative electrode side of the solid-state battery of (4) is mainly made of lithium metal, LiIn alloy, graphite, Li ₄ Ti ₅ O ₁₂ And silicon-containing negative electrodes, the cycle number is not more than 1000 weeks, and the commercial level is difficult to achieve. When lithium metal is used as a negative electrode, the problem of short-circuit failure of the battery caused by dendritic crystal growth exists, research attempts are made on ways such as electrolyte structure design, lithium metal protection and lithium-free negative electrode, but stable long circulation under high current density of a high-load battery cannot be realized, for example, the stable long circulation of the battery is realized by using an Ag-C negative electrode in samsung at 6.8mAh/cm ² Full cell under load and at 3.4mA/cm ² The current density is circulated for 1000 weeks, but the preparation process is more complex; a high load (4 mAh/cm) was observed by Xing Zhang et al ² ) High current density (3.8 mA/cm) of the whole battery at room temperature ² ) Dendrite growth during the next cycle, after 890 cycles, the cell failed by short circuiting. When the 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 limits the performance of the battery, and the energy density is reduced. LTO is very promising for high rate, long cycling (zero strain material), but the high potential limits the voltage and energy density of the battery. ComprisesThe potential of the silicon cathode is proper, the problem of dendritic growth of the lithium metal cathode can be effectively avoided, the interface between the silicon cathode and the electrolyte is relatively stable, and the specific capacity is high (3579mAh/g Li) ₁₅ Si ₄ ) High load, high energy density, high rate are most expected, but the long cycle capacity is limited due to large volume change.

Silicon and sulfide solid-state electrolytes together represent the development direction of the next generation of lithium ion batteries, the electronic conductance of Si (10) ^-3 S/m) and lithium diffusion coefficient (10) ^-14 -10 ^-13 cm ² And/s) is low, the volume expansion is large, active lithium is consumed by repeated generation of SEI in a liquid battery system, the SEI is the main reason of the capacity attenuation of the full battery, and the liquid full battery matched with the silicon cathode has very fast capacity attenuation and is difficult to meet the requirement of a commercial battery. The silicon is used for a sulfide all-solid-state battery system, so that a large amount of interface reaction can be avoided. The volume change of the silicon circulation process is huge, a large amount of lithium is lost in the first interface reaction, and the capacity attenuation of the full battery is aggravated.

Lithium silicon alloy negative electrode (LixSi), compare in lithium metal, can alleviate the growth of lithium dendrite, compare in pure silicon negative electrode, electron conduction and lead lithium ability and all have obvious promotion. Hardness of lithium-silicon alloy (1.3-1.5GPa, Li) _3.75 Si) hardness is less than Si (10.0-11.6GPa), the stress concentration degree of the LixSi alloy negative electrode in the solid-state battery is expected to be less than that of pure silicon, the electrode gradually forms a whole in the circulation process, the generation of pores is reduced, and the material transmission in the electrode is further promoted. Meanwhile, lithium in the LixSi alloy can also supplement lost lithium in the circulating process, so that the circulation is more stable, the volume expansion is smaller than that of pure silicon, and the LixSi alloy is probably 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 conducting 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 proportion parameter of the alloy and the hard carbon, and successfully obtains the all-solid-state battery with high load, large current and long cycle capacity.

The invention provides a safe and stable activated negative electrode which is provided with Li rich in lithium phase ₁₅ Si ₄ 、LiC ₆ And weaving the lithium conductive network in a three-dimensional texture shape.

As an optimized alternative, the three-dimensionally textured lithium conducting network 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 negative electrode can be 1mAh/cm ² Above, can further reach 2mAh/cm ² Above, 3.6mAh/cm ² Above, 4mAh/cm ² Above, 5mAh/cm ² Above, even 6mAh/cm ² The above.

The invention also provides a preparation method of the safe and stable activated cathode, which is 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 arranging the silicon-containing layer and the lithium sheet adjacently to be used as a negative electrode, and activating after assembling the battery.

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

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

As an optimized alternative, the average grain diameter of silicon in the negative electrode after activation is 1.5 +/-1.2 microns, and the average grain diameter of hard carbon is 2 +/-1.5 microns.

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

As an optimized alternative, the pressure level in the battery assembling process is more than 5t, and is preferably 7t-10 t.

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

As an optimized alternative, the battery assembly-matched positive electrode includes, but is not limited to, ternary system (NCM, NCA), lithium cobaltate system (LCO), lithium iron phosphate system (LFP), lithium manganate system (LMO), and the like.

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

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

According to the invention, hard carbon is introduced into the LiSi alloy cathode to prepare the LiSH46 cathode, Si, adjacent Si and C with dangling bonds at the edges of HC are bonded in the circulation process of the battery to generate Si-Si bonds and Si-C bonds, part of crystal boundaries are eliminated to form a uniform and compact electrode, the electrode is subjected to continuous plastic deformation under pressure in the subsequent circulation process, 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 three-dimensional lithium conducting network woven by the method can effectively increase the active area of the electrode and optimize the dynamic performance of the electrode.

The LiSH46 negative electrode can realize high load (3.6 mAh/cm) ² ) High current density (6 mA/cm) ² ) Stable long-cycle (3000 week capacity is kept at 70%), and the thickness change before and after battery cycle is only 2.2%; and ultra high current density (14.64 mA/cm) ² ) Ultra-long cycle (24000 weeks capacity without any decay relative to first week); and may be at 122.1mA/cm ² The silicon-carbon layer can effectively inhibit the growth of lithium dendrites by downward discharge, and the ultra-high surface capacity (16.92 mAh/cm) of the matched anode is realized ² ) The energy density of the battery layer can reach 263 Wh/kg; matched LCO anode at 20C (14.64 mA/cm) ² ) Next, there was no decay in the cycle 24000 weeks capacity; has very practical prospect.

The work takes micron silicon and hard carbon materials as starting points, SH materials are obtained through simple ball milling, LiSH negative electrodes are formed in the process of assembling the battery, the whole process controls the production cost, the safety is improved (the Li-Si alloy is spontaneously combusted in the air), and a new solution is provided for the solid-state battery to be actually applied.

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) testing the multiplying power; (C) charge and discharge curves at different magnifications.

Fig. 2 shows the ionic conductivity and electronic conductivity of materials with different silicon-carbon ratios. (A) Lithium diffusion coefficients of LiSH, Si and SH46 lithium intercalation processes with different silicon-carbon ratios; (B) lithium diffusion coefficients of LiSH and Si lithium removal processes with different silicon-carbon ratios; (C) electrode electron conductance measured by voltammetry.

Fig. 3 is an XRD of the finished LiSi cathodes with different Li/Si capacity ratios.

FIG. 4 shows NMC 811. sup. | LiSi full cell at 1C (6 mA/cm) ² ) Lower cycle, Li/Si capacity ratios of 0.4, 0.6 and 0.8, respectively.

Fig. 5 shows the surface topography of the Si and LiSi cathodes: (A) si; (B) and LiSi.

Fig. 6 shows the electrical properties of a LiSC electrode made by adding graphite, soft carbon and hard carbon to a LiSi negative electrode, respectively, and made into a full cell: (A) cycle testing was performed at 1C; (B) testing multiplying power; (C) different multiplying power charge-discharge curves of the NMC811 LiSG46 full cell; (D) NMC811 LiSS46 full cell different rate charge-discharge curves; (E) NMC811| LiSH46 full cell different rate charge and discharge curves.

Fig. 7 is a full cell with NMC811 positive matched LiSH negative with different Si and hard carbon ratios: (A) carrying out cycle testing; (B) testing the multiplying power; (C) charge and 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 SH46 and LiSH46 negative SEM: (A) surface SEM of SH46 film; (B) LiSH46 negative electrode surface SEM.

Fig. 10 is a sectional view of LiSH46 negative electrode: (A) an initial state; (B) after activation; (C) cycle at 1C for 10 weeks.

Fig. 11 is a structural characterization of LiSH 46: (A) the XRD pattern of LiSH46 was compared with LiHC, LiSi, SH 46. XPS spectrum of LiSH46 negative electrode: (B) c1 s, (C) Li 1 s.

Fig. 12 is an AES characterization of LiSH46 electrodes. (A) Secondary electron imaging; (B) distribution of Li element; (C) c element distribution; (D) and (4) distribution of Si element.

Fig. 13 shows XPS composition analysis of the LiSH46 negative electrode-electrolyte interface: (A) s2 p; (B) cl 2 p; (C) li 1 s.

FIG. 14 shows NMC811| LiSH46 full cell at 1C (5.86 mA/cm) ² ) Current density lower cycle: (A) full battery cycle testing; (B) charge and discharge curves for different cycle numbers.

FIG. 15 shows LiSH46| NMC811 full cell and area capacities above 1mAh/cm in this work ² Comparing the cycle number of the all-solid-state battery.

Fig. 16 shows LCO | LiSH46 full cell: cycling (a) at 20C for the number of cycles and the corresponding specific capacity and coulombic efficiency; (B) a charge-discharge curve; cycling (C) at 30C for the number of cycles and the corresponding specific capacity and coulombic efficiency; (D) different charging and discharging curves.

Fig. 17 shows that the LCO | LiSH46 battery was charged for 20C for one week after 30000 cycles, and returned to 20C cycle after 0.2C discharge.

Fig. 18 shows LCO | LiSH46 full cell versus full solid state cell with more than 1000 cycles in this operation.

Fig. 19 shows NMC811| LiSH46 full cell: (A) multiplying power test (B) NMC811 theoretical surface capacity of 6.48mAh/cm ² (ii) a (C) NMC811| LiSH46 full cell fast discharge test; (D) the NMC811 LiSH46 full cell fast discharge current density is compared to an all solid state cell that exceeds the load of a conventional lithium ion battery.

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

Fig. 21 shows a full cell with a cathode of LiSH46 matched with an ultra-high load NMC 811: (A) the surface loading amounts are 59.83mg/cm respectively ² ,77.17mg/cm ² And 100.12mg/cm ² The charge-discharge curve of the NMC811| LiSH46 full cell at 0.05C; (B) the surface loading is 10.54mAh/cm ² The NMC811| LiSH46 full cell of (1) was charged at 0.05C, and the discharge rate increased from 0.1C to 7.5C; (C)12mAh/cm ² High load NMC811 matched LiSH46 full cell cycling at 0.1C; (D)12mAh/cm ² The high load NMC811 matched the pure Si negative full cell cycle at 0.1C.

Fig. 22 shows the NMC811| LiSH46 full cell load compared to the literature reported solid state cell load.

Fig. 23 shows a LiSH46| NMC811 high load battery operating temperature interval.

Detailed Description

In order that the invention may be more fully understood, preferred embodiments of the invention are now described. It should be understood that these examples are for illustrative purposes only and are not to be construed as limiting the invention in any way, i.e., not intended to limit the scope of the invention.

Preparation of materials

Preparation of negative active material: micrometer Si (3-5 μm, supplied by Tianmu leader) and hard carbon are 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, and are subjected to sealed ball milling for 10 hours, so that obtained samples can be respectively expressed as Si, SH82, SH64, SH46, SH28 and HC, and an argon atmosphere is maintained in the whole process. Control of the Si to carbon mass ratio to 4:6, replacement of hard carbon with soft carbon and graphite produced comparative samples denoted SS46 and SG46, respectively. With samples made with these different silicon and carbon materials at different mass ratios, further batteries were made as LiSi, LiSH82, LiSH64, LiSH46, LiSH28, LiHC, LiSS46 and LiSG46, respectively. The soft carbon and the hard carbon are both purchased from Zhongke Haina company, and D50 is 5-10 mu m.

Preparation of a silicon-containing layer: and mixing the sample obtained by ball milling in the last step with PTFE according to the mass ratio of 95:5, and rolling by using a rolling machine to obtain a film layer, namely the silicon-containing layer. In the high load NMC cell, the quality of the silicon-containing layer was controlled so that the capacity was substantially consistent, the specific capacity of each sample was also measured from the half cell (table 1), and the assembly and testing of the half cell was seen in 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 corresponding Si-containing layer masses

Preparation of NMC811 positive electrode: the preparation method comprises the following steps of fully and uniformly mixing NMC811@ LATP, LPSCl and VGCF at a mass ratio of 80:17:3, adding PTFE to a mixed positive electrode at a mass ratio of 400:3, rolling and folding for multiple times at 80 ℃ after mixing to obtain a positive electrode film with uniform thickness, and punching into a sheet with the diameter of 8mm for later use.

Preparing an LCO positive electrode: and (3) fully and uniformly mixing LCO and LPSCl in a mass ratio of 60:40, adding PTFE into the mixed positive electrode in a mass ratio of 100:0.5, rolling into a positive electrode film with uniform thickness after mixing, and punching into a 9mm sheet for later use.

Structure and characterization of materials

The material phase characterization adopts an X-ray powder diffractometer with the model of XD-2X of Beijing Pujingyo general instrument responsibility Co., Ltd, the testing angle is 10-80 degrees, and the sweeping speed is 8 degrees/min.

SEM characterization of the material was performed using a scanning electron microscope, model SU8100, from Hitachi, Japan.

XPS characterization of the material used PHI5000 Versa Probe III from ULVAC-PHI. Monochromatic Al K α rays were used to study the chemical composition of the negative electrode.

The AES characterization of the material adopts JEOL JAMP-9510F to research the distribution of Li, Si and C elements on the section of a negative electrode.

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

Battery assembly and electrochemical testing

Preparing a composite silicon cathode: mixing the ball-milled negative active material with LPSCl (Li) ₆ PS ₅ Cl) is ground and uniformly mixed in a mortar, and then a conductive agent VGCF is added and uniformly mixed 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: pre-pressing 80mg of electrolyte powder at 1t, uniformly dispersing 2mg of composite cathode powder on one side, maintaining the pressure for 3min at 7t, sequentially attaching an In foil and an Li foil on the other side, wherein the molar ratio of In to Li is more than 1, applying 1t of pressure and simultaneously screwing screws to obtain the solid-state half cell. The cell was tested at a set temperature (e.g., 55 deg.C) with a test voltage range of-0.615-0.88V and a test current density of 400 mA/g.

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

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

Electrochemical testing: the load was 30mg/cm ² The NMC811 battery is activated after being circulated for 2 weeks at 0.1 ℃, and then circulation and multiplying power tests are carried out, wherein the testing multiplying power of the circulation test is 1C (-6 mA/cm) ² ) And circulating for 5 weeks under the conditions of 0.2C, 0.5C, 1C, 1.25C, 1.5C, 1.75C, 2C and 0.1C in a multiplying power test respectively. The test multiplying power of the ultra-high-load NMC811 battery is 0.05C, and the cycle test is carried out at 0.1C. LCO cells were cycled for one week at 0.2C and 0.5C, and then cycled and rate tested at a rate of 20C (14.64 mA/cm) ² )、30C(20.45mA/cm ² ) The magnification test is 5 weeks for each cycle at 1C, 5C, 10C, 15C, 20C, 30C, 40C, 50C, 1C.

Assembling the PITT battery: taking 80mg of electrolyte powder, prepressing at 1t, sequentially placing a silicon-containing layer and a Li sheet at one side, keeping the pressure at 7t for 3min, sequentially placing an In sheet and a Li sheet at the other side, and screwing screws of the battery.

PITT test: the battery is kept still for 2h, constant voltage is started from open-circuit voltage, voltage interval is set to be 5mV, the voltage is constant until the current is less than 2 muA 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).

Fourth, result verification and analysis

4.1 verification of Performance optimization results of Si, LiSi and LiSC cathodes

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), and the capacity of the negative electrode made of the LiSi alloy is almost not decayed, and micro short circuit occurs after several weeks of circulation. The rate test is carried out on the battery (figure 1B), when the current density is increased to 0.5C, the battery has serious micro short circuit, and the micro short circuit phenomenon can be effectively relieved by introducing Hard Carbon (HC) LiSC into LiSi; while LiSC also maintains stable long cycling of the cell at high load, high current density, as in fig. 1C.

The experiments preliminarily prove that the formation of the lithium-silicon alloy effectively improves the lithium conducting capability of the pure silicon cathode, and is a basic guarantee for keeping higher reversible capacity. The lithium silicon alloy cathode is obtained by spontaneous reaction after lithium is embedded in the silicon cathode, a lithium source can be from a metal lithium sheet, and the lithium silicon alloy cathodes with different Li/Si capacity ratios can be obtained by adjusting the mass of the metal lithium sheet and the Si content in the silicon-containing layer. The Li/Si capacity ratio (mass of Li + specific capacity of Li)/(mass of Si + specific capacity of Si) ═ Li/Si-containing layer capacity ratio (mass of Li + specific capacity of Li)/(mass of Si-containing layer specific capacity of Si-containing layer), where the specific capacities may be taken separately during calculation: the specific capacity of Li is 3860mAh/g, and the specific capacity of Si is 3300 mAh/g. The lithium silicon alloy negative electrode is denoted as LixSi or LiSi in the present invention.

The volume change of the pure silicon cathode is large in the process of lithium extraction and insertion (Li) ₁₅ Si ₄ Volume expansion is 280%), a large number of cracks are easily generated in the electrode, partial contact loss is caused, meanwhile, the passivation layer on the electrode electrolyte interface is continuously broken and generated due to volume change, the passivation layer becomes thick, partial active silicon is insulated, a large amount of lithium is lost, electrode polarization is increased, and battery capacity is continuously attenuated. From the kinetic point of view, as shown in FIG. 2, the lithium diffusion coefficient is the smallest at 10 for pure silicon intercalation ^-15 m ² S, electron conductance is also only 10 ^-6 S/m, needs to be larger thanThe potential drives the lithium intercalation reaction, the electrode polarization is large, and the reversible capacity of the pure silicon cathode under large current is obviously lower than that of LiSi and LiSH 46. The capacity fading of the full battery rate test matched with the pure Si cathode is obviously faster than that of the cycle test because the SOC of a small current is larger than that of a large current process, the volume change degree of silicon is larger than that of the large current process, more cracks can be generated in the electrode, and the capacity fading is aggravated.

Si and Li can spontaneously react to generate amorphous LixSi alloy, and the amorphous LixSi alloy is transformed into crystalline Li during full intercalation ₁₅ Si ₄ The Young's modulus decreases after Si intercalation, and Li is fully intercalated ₁₅ Si ₄ Has a hardness of 1.3-1.5GPa, which is about an order of magnitude smaller than that of Si (10-11.6GPa), Li ₁₅ Si ₄ The stability of the electrode can be improved. Li ₁₅ Si ₄ Has a lithium diffusion coefficient of 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 The lithium ion diffusion coefficient of Cl was 2.5X 10 ^-12 m ² In/s). The electronic conductance of the LiSi alloy is also significantly improved compared with that of Si, Li ₁₅ Si ₄ Electrode dynamics can be significantly enhanced. As shown in FIG. 2, the electron conductance of the LiSi anode (0.87S/cm) was compared to that of pure silicon (8.8X 10) ^-6 S/cm) is improved by 5 orders of magnitude, and the lithium ion diffusion coefficient is 10 ^-14 m ² There is also a boost (about 2 times) in/s). During the pressing process of the battery, Si and the lithium sheet 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 Has a significant broadening of the peak of (A), and also has a significant Si peak, Li at a Li/Si capacity ratio of 0.8 ₁₅ Si ₄ The peak of (a) is more pronounced and the peak of Si is not pronounced, indicating that the crystal structure of Si in the electrode is destroyed and that no larger intact Si particles are present.

From the 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 structure is free from Si large particles, electrode pulverization or interface passivation layer repeated generation is caused, and stable long circulation of the electrode is easy to realize. Respectively using LiSi alloy cathodes with Li/Si capacity ratios of 0.4, 0.6 and 0.8 to match 6mAh/cm ² Full electricity of NMC811Cell, and at 1C (. about.6 mA/cm) ² ) When the Li/Si capacity ratio was 0.8, the reversible capacity of the full cell was about 120mAh/g, and almost no fading occurred in 27 cycles, and short-circuiting occurred in 27 th cycle, as shown in FIG. 4. While 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 the silicon-carbon composite silicon-containing layer is used to replace pure silicon cathode in the present invention, the silicon-containing layer can be designed with the same capacity to maintain the same cycling capability, i.e. the capacity ratio of Li/silicon-containing layer is preferably above 0.6, more preferably 0.8 + -0.1.

The cause of short circuits after cycling of the LiSi anode may be: 1) the LiSi negative electrode has a limited lithium diffusion coefficient and when a large current is applied to the electrode, lithium may be deposited at the electrode-electrolyte interface rather than diffusing into the interior of the electrode. 2) As shown in fig. 5, the LiSi negative electrode is dense in surface and has no pores similar to the Si thin film, the Si expands in volume after lithiation, the pores of the electrode decrease, excess lithium cannot be accommodated, and lithium gradually accumulates during battery cycling and penetrates through the electrolyte, causing a short circuit of the battery. 3) Lithium begins to precipitate after the silicon full intercalation, the nucleation potential of the silicon surface is high, large dendrites tend to form, and therefore the LiSi alloy negative electrode is prone to micro short circuit.

The carbon material can improve electrode dynamics, can provide defects and pores to store excessive lithium, and is expected to solve the short circuit problem of the LiSi negative electrode. Carbon materials are a wide variety of materials, including graphite, soft carbon, and hard carbon. Wherein, hard carbon refers to carbon which can not be graphitized at the temperature of more than 2500 ℃, and is mainly prepared by resin matrix, asphalt matrix and biomass matrix precursors; the soft carbon refers to carbon capable of being graphitized at 2500 ℃ or higher, and its soft carbon precursor may include petroleum coke, petroleum pitch and condensed ring aromatic compounds. The graphite, the soft carbon and the hard carbon have obvious differences in the aspects of structure order degree, interlayer spacing, structure 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 full cell, results are shown in fig. 6. The introduction of graphite and soft carbon can not maintain the high reversible capacity of the full cell under large currentIn the presence of micro short circuit, the Hard Carbon (HC) can stabilize the lithium silicon cathode, realize stable long circulation under high current density, and when the current density reaches 12.96mA/cm ² (2C) When the cell did not develop significant micro-shorts. HC has abundant defects and holes, the lithium intercalation voltage range is 0.8-0.001V, which is close to 0V, lithium precipitated after LiSi alloy full intercalation can be intercalated into HC and then precipitated from the surface of HC, because of smaller potential difference between the full intercalation HC and the metal lithium, the precipitation rate of the metal lithium is slower, the precipitated lithium tends to be uniformly deposited on the surface of an electrode, HC can effectively inhibit the growth of lithium dendrites, HC can also play a role in supporting, HC has almost no volume change in the lithium deintercalation process, the volume change of Si can be buffered, and the stability of the electrode is maintained. HC can effectively improve electrode dynamics, and as seen from the PITT test result (figure 2), the addition of HC obviously improves the lithium diffusion coefficient of the electrode, improves the lithium diffusion coefficient of the electrode by at least one order of magnitude compared with Si and LiSi, and as the proportion of HC increases, the lithium diffusion coefficient of the electrode increases, but the thickness of the electrode increases (table 1), the migration path of lithium becomes longer, and the diffusion time (t ℃. L) ² ) And (4) increasing.

Effect of the ratio of Si and HC on Battery Performance As shown in FIGS. 7(A-C), the LiSH46 negative electrode exhibited the best performance for an all-solid-state battery at 1C (6 mA/cm) when the Si and HC mass ratio was 4:6 ² ) Maintain a reversible capacity of 123.6mAh/g at 2C (12.96 mA/cm) ² ) No micro-short occurs. When Si is excessive, the battery faces micro short circuit under large current, the battery is similar to a LiSi negative electrode, under large current, the lithium diffusion coefficient is insufficient, lithium is fully diffused into an electrode, lithium dendrite grows on an interface deposition, the Coulomb efficiency of the LiSH82 negative electrode is low, obvious micro short circuit continuously occurs, lithium deposited by the electrode continuously reacts with electrolyte, interface impedance is continuously increased, and battery capacity is rapidly attenuated, in the circulating process of the LiSH64 negative electrode, the Si volume is greatly changed, 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 grows gradually, lithium dendrite reacts with the electrolyte, more active lithium is consumed, and the battery capacity is rapidly attenuated after stable circulation for a plurality of weeks. When HC is excessive, the lithium diffusion coefficient of the electrode is improved, but that of the electrodeThe thickness is remarkably increased (LiHC-170 mu m, LiSH 28-70 mu m), the diffusion time of lithium in the electrode is remarkably increased, the utilization rate of the electrode is reduced under large current, and the reversible capacity of the battery is lower. The electrode is susceptible to diffusion-controlled lithium trapping, resulting in a large amount of lithium being trapped inside the electrode and a continuous decline in battery capacity. Therefore, optimizing the ratio of Si and HC can further improve the large current and long cycle capability of the lithium silicon alloy negative electrode. The proportion of the mass of Si in the total mass of Si and HC may be 20% to 80%, preferably 30% to 60%, most preferably 40% ± 5%.

4.2 characterization and optimization theoretical analysis of hard carbon stabilized lithium silicon alloy negative electrode

The LiSH46 negative electrode is formed by pressing SH46 powder and PTFE roll into an SH46 thin film, and then is in short circuit contact with a Li sheet and reacts under pressure. The morphology of SH46 powder is shown in FIG. 8, HC and Si are both micron-sized particles below 8 μm, HC has an average particle size of 2.39 μm, and Si has an average particle size of 1.21 μm. The prepared SH46 film has a thickness of 33 μm, and when viewed from the surface, as shown in FIG. 9, the particles show the characteristic of crystal self-plasticity and have a more regular structure. When a SH46 thin film and a 35 μm lithium sheet were used to form a LiSH46 negative electrode, the electrode thickness became 45 μm (fig. 10A), and LiSH46 became denser than SH46, and some of the grain boundaries and pores disappeared, similar to the phenomenon of the LiSi negative electrode. LiSH46 was activated by cycling at 0.1C for 2 weeks, the electrode thickness became 46 μm, and the electrode showed no significant transverse cracks when viewed in cross-section (FIG. 10B), demonstrating the stability of the LiSH46 structure.

LiSH46 negative electrode contains Li ₁₅ Si ₄ And LiC ₆ As shown in fig. 11A, Li is generated in the LiSH46 negative electrode ₁₅ Si ₄ Phase, in contrast to LiHC negative electrodes, Li ₁₅ Si ₄ Main peak of (3) and LiC ₆ The main peaks of (a) coincide. Peak fitting of XPS C1 s spectra of LiSH46 negative electrodes (fig. 11B) demonstrated the production of LiC in LiSH46 negative electrodes ₆ (285.2 eV). Peak-splitting fitting was performed on the Li 1s spectrum (FIG. 11C), and the generation of Li-C (56.2eV) and Li-Si (55.4eV) was also confirmed. The existence of the lithium-rich phase greatly improves the electronic conductance and the lithium diffusion coefficient of the electrode and the electrode dynamics, and as shown in the results of the PITT test and the voltammetry test of FIG. 2, the lithium diffusion coefficient and the electrons of the LiSH46 negative electrodeThe electrical conductivity is obviously improved compared with pure Si and LiSi cathodes.

AES characterization is carried out on the activated LiSH46 electrode, distribution of Li, Si and C elements on the cross section is analyzed, the result is shown in figure 12, the Si and C elements lose original regular shapes and are changed into texture shapes, enrichment sites of the C and Si elements are adjacent, the Li elements are distributed in the electrode and basically coincide with the enrichment sites of the Si and C elements, and the result shows that lithium-rich phases in the LiSH46 negative electrode are woven into a three-dimensional conductive lithium-conducting network. The battery is circulated under external pressure, Si is amorphous and plastically deforms when lithium is embedded, and the graphite-like layer of HC particles is pushed to migrate along with volume expansion, microcracks caused by the volume expansion of Si are relieved by the strong constraint capacity of the HC particles relative to SC, Si, adjacent Si and C with dangling bonds at the edges of HC are bonded in the lithium removal process to generate Si-Si bonds and Si-C bonds, partial crystal boundaries are eliminated, a uniform and compact electrode is formed, and the electrode continuously deforms plastically under pressure in the subsequent circulation process, so that the electrode is driven to form a uniform and almost stress-concentration-free continuous body, and the stability of the electrode is enhanced. The three-dimensional lithium conducting 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 LiSH46 negative electrode and electrolyte interface was also confirmed by XPS, as shown in FIG. 13 by the S2 p region peak-to-peak fit results, and a small amount of electrolyte on the surface of the LiSH46 negative electrode was decomposed and Li was added to the cell as it was produced ₂ PS of S (159.9eV) formation, electrolyte ₄ ^3- (161.2eV) was also the main phase and was not significantly reduced after activation, indicating that a relatively stable interfacial passivation layer was generated on the surface of the LiSH46 negative electrode. Peaks after Cl 2p activation and in the initial state represent Cl ^- (198.4eV), there was no difference. The P2P peak-to-peak fit results show only a small amount of PS ₄ ^3- Is reduced to P ₂ S ₇ ^4- . And a composite negative electrode or a lithium metal negative electrode containing the electrolyte reported in the literature, the electrolyte decomposition degree of the LiSH46 surface is weak, and the interface stability of the LiSH46 negative electrode and the electrolyte is demonstrated.

The above results and analysis demonstrate that the hard carbon material provides intercalation and deposition of metallic lithiumAbundant storage space is favorable to fully holding metal lithium and avoiding lithium dendrite, and when being favorable to quick ion diffusion, still has the supporting role better for soft carbon material, is favorable to alleviating the huge volume expansion deformation of Si through the great migration restriction between the layer. The amorphous LiSi alloy promotes the refinement of Si particles, improves the contact among the particles, and precipitates and derives nearby lithium-rich phases, so that a three-dimensional conductive lithium-conducting network woven by the lithium-rich phases is formed in the negative electrode, the three-dimensional conductive lithium-conducting network effectively increases the ion conductivity of the negative electrode, and the electrode dynamics is improved. It can therefore also be concluded that large-grained crystalline Si, which is not expected to remain too much in the negative electrode system, is expected to be more transformed to amorphous morphology and partly to Li ₁₅ Si ₄ And LiC ₆ To build an effective three-dimensional ionic conduction network in the interface, other means that can bring about such conversion can be utilized for the same reasons and achieve substantially the same desired effect. For example, we can introduce amorphous LiSi alloy, Li, directly into hard carbon particles in the desired stoichiometric ratio ₁₅ Si ₄ And LiC ₆ And (4) coating is carried out, so that a three-dimensional ionic conduction network is formed more directly.

4.3 full cell electrochemical performance improvement verification under high load, large current and high multiplying power

To further characterize the performance of the LiSH46 electrode in a full cell, this work matched NMC811 and LCO anodes respectively to demonstrate the superior electrochemical performance of LiSH 46. The NMC811 composite cathode is used for constructing a high-load full cell, and the active substance accounts for 80 percent. The LCO composite cathode is loaded in a smaller mode, the influence of diffusion length is eliminated, the LCO composite cathode is used for testing the performance of the LiSH46 cathode under large current and high rate, and the active substance accounts for 70%.

Match high load (5.89 mAh/cm) ² ) NMC811 electrode with high active material content (80%) at 1C (5.89 mA/cm) ² ) Circulating under current density (figure 14), the reversible capacity can reach 123.6mAh/g (3.6 mAh/cm) ² ) The lithium ion battery reaches the level of a commercial lithium battery, the cycle life reaches 1033 circles, 70 percent of reversible capacity is still maintained after 3000 circles of circulation, and the battery capacity is mainly attenuatedFor the first 100 weeks, capacity declined by 10%, and 3000 cycles retained 80% compared to week 100. The cycle number of the battery is that the surface capacity reported by the current literature is higher than 1mAh/cm ² The longest of the all-solid-state batteries (FIG. 15) exceeds the previously reported μm-Si negative electrode (area capacity of 2 mAh/cm) ² At 5mA/cm ² Lower cycle for 500 weeks), Ag-C cathode (surface capacity of 4.62 mAh/cm) ² At 3.4mA/cm ² 1000 cycles of bottom cycle) and a LiIn negative electrode (surface capacity of 4 mAh/cm) ² At 3.8mA/cm ² 897 cycles for the lower cycle, short circuit failure occurred). The excellent cycling stability of LiSH46 can be attributed to the structural stability, the LiSH46 negative electrode lithium-rich conductive sub-network is easy to generate plastic deformation under stress, the stress concentration in the electrode is reduced, and the stability of the electrode is maintained, as shown in figure 10(C), after the electrode is cycled for 10 weeks at 1C, no obvious transverse crack exists, the structural stability is obviously superior to that of the mum-Si reported by Meng and the like, and the transverse crack is very obvious after the electrode is charged and discharged for the first time. The structure of the LiSH46 negative electrode is stable while maintaining the stability of the two-dimensional passivation layer between the electrode electrolytes. The electrode exhibits excellent cycle stability.

TABLE 2 NMC811| LiSH46 cell of this work and literature reports of surface capacities higher than 1mAh/cm ² Solid state battery cycle count comparison

To further demonstrate the cycling stability of the LiSH46 negative electrode, we reduced the thickness of LiSH46 to match it to 5mg/cm at high loads where there was too long a diffusion path to limit cell performance ² Left and right LCO positive electrodes respectively corresponding to the positive electrodes 20C andthe current density was 14.64mA/cm for each long cycle at 30 ℃ in FIG. 16 ² And 20.45mA/cm ² And the initial reversible capacity at 20 ℃ is 76mAh/g, the capacity gradually rises to 87mAh/g in the circulation process and then falls back to 80mAh/g, the capacity is obviously attenuated from 24000 circles, and the capacity is circulated to 30000 circles, and the capacity keeps 72.1 percent of the initial reversible capacity. The initial reversible capacity at 30C was 52.06mAh/g, the reversible capacity rose to 73mAh/g (-10000 weeks) during cycling, and then the capacity continued to fade to 80% of the initial capacity at 15000 cycles. As shown in fig. 16B and (D), as the battery cycles, the polarization of the battery decreases and the battery capacity increases, for two reasons: 1) in the circulation process, Si-Si and Si-C bonds eliminate a part of crystal 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 a large current, lithium diffusion and electron conduction inside the electrode are promoted, and the battery capacity continuously increases.

The capacity of the battery subsequently decayed, partly due to diffusion-controlled lithium capture, as shown in fig. 17, the battery cycled at 20C for 30000 weeks was discharged at 0.2C with a discharge capacity of 99.11mAh/g, lithium captured in the LiSH46 negative electrode was extracted and subsequently returned to 20C constant current charging and discharging, the capacity of the battery increased from 55mAh/g at 30000 weeks to 66.12mAh/g, and partial capacity was returned, indicating that lithium capture in the negative electrode caused partial capacity loss. The major capacity loss may also be due to electrode structure failure, requiring further investigation. The cycle life of the LCO | LiSH46 full cell as shown in fig. 18 exceeds that of all full solid state cells.

Table 3 LCO | LiSH46 cells of this work compared to the number of cycles reported in the literature for solid state cells cycling over 1000 cycles

Full cells matched to the LiSH46 negative electrode showed very excellent stability at ultra-high current densities. As shown in FIG. 19, match 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 are carried out at a current density exceeding that of near-loaded μm-Si (5 mA/cm) without occurrence of significant micro-short-circuiting ² ) Or lithium metal negative electrode (Ag-C3.4 mA/cm) ² ，LiIn 3.8mA/cm ² ) The full cell of (3). The battery can realize discharge at the maximum of 20 ℃, and the current density reaches 122.1mA/cm ² The present flour capacity exceeds 1mAh/cm ² The highest discharge current in the all-solid-state battery of (2). The full battery matched with the LiSH46 negative electrode can realize ultrahigh current density, mainly because of the three-dimensional lithium-rich lithium-conducting network in the electrode, compared with the two-dimensional reaction interface of the Si negative electrode 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 rate rather than being deposited on the electrode electrolyte interface.

TABLE 4NMC811 LiSH46 Total cell Current Density and literature reported surface Capacity higher than 1mAh/cm ² For comparison of solid state batteries.

To exclude the effect of diffusion paths, and to further demonstrate 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, the full battery can correspond to the positive electrode 50C (71.5 mA/cm) at ultrahigh multiplying power ² ) The highest current density (8.6 mA/cm) of the lithium metal battery that could be achieved by the multilayer solid electrolyte reported by Li et al was achieved by charging and discharging without occurrence of a micro short (FIG. 20) ² ) 8.3 times of the total weight of the powder. At the corresponding positive electrode 10C (6 mA/cm) ² ) The lower full cell maintained a reversible capacity of 103mAh/g (0.2C reversible capacity of 137mAh/g), fully demonstrating the excellent rate capability and stability of LiSH46 at ultra-high current densities.

To verify LiSH46 negative pole tolerance to lithium metal, 6mAh/cm for this work ² The LiSH46 negative electrode of the full cell is matched with the ultra-high load NMC811 positive electrode, as shown in FIG. 21, when excessive lithium migrates to the negative electrode side, the negative electrode side is converted into a lithium metal protective layer, a large amount of lithium is deposited on the negative electrode side without micro short circuit, and 6mAh/cm ² LiSH46 can bear at least 20mAh/cm ² The reversible charge and discharge of the high-load positive electrode NMC811 shows the inhibition effect of LiSH46 on the growth of lithium dendrites, and the LiSH46 negative electrode is not easy to grow the lithium dendrites in the actual use process. We used 6mg/cm ² The LiSH46 negative electrode realizes the highest reversible surface capacity of 16.92mAh/cm reported by the current all-solid-state battery ² And (figure 22), the surface capacity of the system has a further improved space, the mass of the negative electrode can be improved, the N/P ratio can be increased to match the positive electrode with higher load, the active material ratio can be reduced for the positive electrode, and the internal ion transport of the electrode is optimized to further improve the battery capacity. As shown in FIG. 21C, the positive electrode load was 12mAh/cm ² The LiSH46| NMC811 full cell can be at 1.22mA/cm ² Realize stable circulation and keep close to 9mAh/cm ² The coulombic efficiency is higher than 90%, while the pure silicon cathode full battery capacity of the same load is rapidly attenuated, as shown in figure 21D, the capacity is only kept at 3mAh/cm in the 10 th week ² Coulomb efficiency was low, and micro-short occurred by cycling to week 12. As shown in Table 6, the cell level energy density of the NMC811| LiSH46 full cell was close to the liquid cell level, 20mAh/cm ² The energy density of the cell layer (positive electrode + negative electrode + electrolyte layer) of the high-load NMC811| LiSH46 full cell is as high as 263Wh/kg, the cell uses the 650 mu m thick electrolyte layer, the energy density can be further improved by thinning the electrolyte layer, and if the 20 mu m thick electrolyte layer is used, the cell energy density can be improved to 450Wh/kg, and the cell system has great application prospect.

TABLE 5 NMC811| LiSH46 full cell load and literature reports full cell comparisons

Table 6 summary of LiSH46 NMC811 battery energy densities for 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, the capacity of the battery at low temperature is low because the positive active substance accounts for too high (80%), the conductivity of the conductive path in the electrode is obviously reduced at low temperature, and the capacity of the positive electrode is difficult to exert. From the discharge curve, the solubility plateau capacity of lithium increases in proportion to the overall capacity as the temperature decreases, indicating that li insertion by LiSH46 is somewhat limited at potentially low temperatures.

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The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.

Claims (10)

1. A safe and stable activated negative electrode with Li-rich phase ₁₅ Si ₄ 、LiC ₆ And weaving the lithium conductive network in a three-dimensional texture shape.

2. The activated negative electrode of claim 1, wherein the lithium conducting network with three-dimensional texture is distributed in amorphous lithium silicon alloy.

3. The activated negative electrode according to claim 1, wherein 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.

4. The activated negative electrode of claim 1, wherein the negative electrode has a load of 1mAh/cm ² Above, 2mAh/cm ² Above, 3.6mAh/cm ² More than 4mAh/cm ² Above, 5mAh/cm ² Above, or 6mAh/cm ² The above.

5. The method for preparing a safe and stable activated negative electrode according to any one of claims 1 to 4, wherein the silicon-containing layer is prepared by mixing and grinding silicon particles and hard carbon particles, and then mixing the mixture with a binder; and arranging the silicon-containing layer and the lithium sheet adjacently to be used as a negative electrode, and activating after assembling the battery.

6. The method for preparing a safe and stable activated negative electrode according to claim 5, wherein the ratio of the mass of silicon in the negative electrode to the total mass of silicon and hard carbon is 20% to 80%, preferably 30% to 60%, and most preferably 40% ± 5%.

7. The method for preparing the safe and stable activated negative electrode according to claim 5, wherein the raw materials of silicon and hard carbon in the negative electrode are micron-sized particles with the average particle size of below 8 μm;

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

8. The method for preparing the safe and stable activated cathode according to claim 5, wherein the silicon-containing layer has a specific capacity of 900-3000 mAh/g and a thickness of 5-100 μm; the preferable specific capacity is 1500-2500 mAh/g, and the thickness is 15-40 mu m.

9. The method for preparing a safe and stable activated negative electrode according to claim 5, wherein the pressure during the battery assembly process is 5t or more, preferably 7t to 10 t;

the cyclic activation is carried out at a rate of 0.1C-0.5C for at least 1 week, preferably 1-2 weeks.

10. A battery comprising the safe and stable activated anode of any of claims 1-4.

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