WO2017214882A1

WO2017214882A1 - Porous silicon particles and a method for producing silicon particles

Info

Publication number: WO2017214882A1
Application number: PCT/CN2016/085848
Authority: WO
Inventors: Jun Yang; Rongrong MIAO; Xiaolin Liu; Yuqian DOU; Jingjun Zhang; Rongrong JIANG; Lei Wang; Xiaogang HAO; Qiang Lu
Original assignee: Robert Bosch Gmbh
Priority date: 2016-06-15
Filing date: 2016-06-15
Publication date: 2017-12-21
Also published as: CN107848809B; CN107848809A

Abstract

Provided are a porous silicon particles, a method for producing the same, a silicon-carbon composite, an electrode material and a battery comprising the composite, a method for producing the battery, and the use of the silicon-carbon composite as an electrode active material. The method for producing silicon particles includes the following steps: 1) preparing a mixture of a silica source material, magnesium and/or aluminium powder as a reducing agent, and a salt or composite salt as a heat absorbent; 2) heating the mixture obtained from step 1) at a heating temperature of from the melting point of the reducing agent to lower than 800 ℃ under a protective atmosphere; 3) removing the heat absorbent and the oxidation products of the reducing agent; characterized in that the melting temperature of the salt or the liquidus temperature of the composite salt ranges from a temperature higher than the heating temperature of step 2) to 800 ℃.

Description

POROUS SILICON PARTICLES AND A METHOD FOR PRODUCING SILICON PARTICLES

Technical Field

The present invention relates to porous silicon particles, a method for producing silicon particles, a silicon-carbon composite, an electrode material and a battery comprising said composite, a method for producing said battery, and the use of the silicon-carbon composite as an electrode active material.

Background Art

As the exigent demand for high energy lithium ion batteries (LIB) used in portable devices, (hybrid) electric vehicles (HEV) and grid-scale stationary energy storage, silicon (Si) has attracted tremendous attention due to its ten times higher theoretical capacity than traditionally used graphite anodes. However, the main impediment of silicon as anode material is huge volumetric change during repeated lithiation/delithiation processes. Repeated huge volumetric change leads to Si pulverization, cracks of electrode and continuous solid electrolyte interface (SEI) growth, which lead to loss of electronic and ionic conductivity. To address such a problem, extensive research has been devoted. For instance, silicon with nano-scaled size could buffer the large (de) lithiation strains without fracture to some extent； other well-defined Si nanostructures including nanowires, nanotubes, porous structures and their composites with carbon materials have been proposed to alleviate volume expansion as well. However, except for pursuing long cycling life and excellent specific capacity of silicon, the cost for Si production is also a critical parameter to consider for its widespread application as anode material. As we all know, with the increase of hybrid electrical vehicles (HEVs) and electrical vehicles (EV) markets, the price appears to be another great challenge for LIB production. Therefore, in order to develop large-scale silicon as anode material, choosing cheap raw materials and scalable manufacturing processes is becoming a major focus of recent battery research.

Among the diversity of silicon manufacturing processes, magnesiothermic reduction method is of great potential for large-scalable production based on its cheap raw material Mg powder and simple devices. Diverse silicon with porous structure has been synthesized by magnesiothermic reduction method and exhibits good electrochemical performance. Nevertheless, owing to the exothermic nature of magnesiothermic reduction reaction, massive heat will be released during the reactive process and results in a much higher reaction temperature than the set one. In this case, once the reaction is large-scaled, the architectures of the silica precursor will be easily collapsed and agglomerated products will be simultaneously formed under a too high temperature. Meanwhile, some side reaction products Mg₂Si and Mg₂SiO₄ will significantly influence the electrochemical performance of the synthesized silicon. Therefore, it is highly critical to control the temperature in the magnesiothermic reduction reaction with an efficient but cost-effective method when large scale production is developed. Based on the problem mentioned above, many research groups focus on inducing salts as heat absorber to produce silicon via the magnesiothermic reduction method. Following are some prior art results:

Xianbo Jin, et al., “Electrochemical preparation of silicon and its alloys from solid oxides in molten calcium chloride” , Angewandte Chemie, 2004.116 (6) : p. 751-754 firstly reported the electrochemical preparation of silicon from solid oxides in molten calcium chloride. In this report, the molten CaCl₂ serves as electrolyte at 850℃ and the silica could be directly electroreduced to silicon by fabricating SiO₂ powder into a porous electrode.

Liu, X., et al., “A molten-salt route for synthesis of Si and Ge nanoparticles: chemical reduction of oxides by electrons solvated in salt melt” , Journal of Materials Chemistry, 2012. 22 (12) : p. 5454-5459 reported a molten-salt route for synthesis of Si nanoparticles, in which the LiCl/KCl and NaCl/MgCl₂ eutectic molten salts act as a reaction “solvent” and provided a salt-melt liquid environment in the magnesiothermic reduction of silica. The growth of Si nanocrystals could be controlled by adjusting the temperature and the type of salt.

Luo, W., et al., “Efficient Fabrication of Nanoporous Si and Si/Ge Enabled by a Heat Scavenger in Magnesiothermic Reactions” , Scientific Reports, 2013. 3 reported an efficient method to fabricate nanoporous Si by employing NaCl as heat scavenger in magnesiothermic reactions with a weight ratio of silica to NaCl of 1: 10. The collapse of original porous structure of silica precursor will be minimized by the fusion of NaCl, which consumes a large amount of heat released by the magnesiothermic reaction.

On the other hand, in the effort to design a high-power battery, the reduction of active material particle size to nano-scale or generate porous structure can help shorten the diffusion length of charge carriers, enhance the Li-ion diffusion coefficient, and therefore achieve faster reaction kinetics. However, nano-sized or porous active materials have a large surface area, which results in a high irreversible capacity loss due to the formation of a solid electrode interface (SEI) . For silicon oxide based anode, the irreversible reaction during the first lithiation also leads to a large irreversible capacity loss in initial cycle. This irreversible capacity loss consumes Li in the cathode, which decreases the capacity of the full cell.

Even worse, for Si-based anode, repeated volume change during cycling reveals more and more fresh surface on the anode, which leads to continuous growth of SEI. And the continuous growth of SEI continuously consumes Li in the cathode, which results in capacity decay for the full cell.

In order to provide more lithium ions to compensate for an SEI or other lithium consumption during the formation, additional or supplementary Li may be provided by the prelithiation of the anode. If the prelithiation of the anode is conducted, the irreversible capacity loss could be compensated in advance instead of Li consumption from the cathode. This results in higher efficiency and capacity of the cell.

However, a pre-lithiation degree of exact compensation for the irreversible loss of lithium from the anode doesn’ t help to solve the problem of Li consumption from the cathode during cycling. Therefore, in this case, the cycling performance will not be improved. To compensate for the loss of lithium from the cathode during cycling, an over-prelithiation is conducted in the present invention.

Summary of Invention

Inspired by the above researches, the inventors of the present invention successfully develop a large-scale silicon production process to achieve a high reversible capacity by using an appropriate salt or composite salt as a heat absorbent and using the extremely price advantageous silica precursor in a thermal reduction process.

The present invention provides a method for producing silicon particles, said method including the following steps:

1) preparing a mixture of a silica source material, magnesium and/or aluminium powder as a reducing agent, and a salt or composite salt as a heat absorbent；

2) heating the mixture obtained from step 1) at a heating temperature of from the melting point of said reducing agent to lower than 800℃ under a protective atmosphere；

3) removing said heat absorbent and the oxidation products of said reducing agent； wherein the melting temperature of said salt or the liquidus temperature of said composite salt ranges from a temperature higher than the heating temperature of step 2) to 800℃.

The present invention, according to another aspect, relates to porous silicon particles, which have a bimodal pore size distribution of < 2 nm and 10 –30 nm.

The present invention, according to another aspect, relates to a silicon-carbon composite, which comprises a carbon coating layer as well as the silicon particles according to the present invention.

The present invention, according to another aspect, relates to an electrode material, which comprises the silicon-carbon composite according to the present invention.

The present invention, according to another aspect, relates to a battery, which comprises the electrode material according to the present invention.

The present invention, according to another aspect, relates to the use of the silicon-carbon composite according to the present invention as an electrode active material.

Brief Description of Drawings

Each aspect of the present invention will be illustrated in more detail in conjunction with the accompanying drawings, wherein:

Figure 1 shows the XRD pattern of the silicon particles of Example 1；

Figure 2 shows the cycling performances of the silicon particles of Example 1 (E1) and Comparison Example 1 (CE1) ；

Figure 3 shows the cycling performances of the silicon particles of Examples 1 ～ 7 (E1 ～ E7) and Comparison Examples 1 ～ 3 (CE1 ～ CE3) ；

Figure 4 shows the XRD patterns of the silicon particles of Example 3 (E3) , Example 6 (E6) , Example 5 (E5) , Example 2 (E2) , and Comparison Example 2 (CE2) ；

Figure 5 shows the SEM images of the silicon particles of Comparison Example 2 (CE2) , Example 2 (E2) , Example 5 (E5) , Example 6 (E6) , and Example 3 (E3) ；

Figure 6 shows the XRD patterns of the silicon particles of Example 7 (E7) and Comparison Example 3 (E3) ；

Figure 7 shows (a) the SEM and (b) the TEM images of the silicon particles of Example 8；

Figure 8 shows the N₂-sorption isotherms of (a) the raw material porous SiO₂ and (b) the silicon particles of Example 8；

Figure 9 shows the pore size distribution of (a) the raw material porous SiO₂ and (b) the silicon particles of Example 8；

Figure 10 shows the cycling performances of the silicon particles of Examples 8 (E8) and the silicon-carbon composite of Example 9 (E9) ；

Figure 11 shows the rate capability of the silicon-carbon composite of Example 9；

Figure 12 shows the cycling performances of the full cells of Example P1-E1；

Figure 13 shows the normalized energy densities of the full cells of Example P1-E1；

Figure 14 shows the cycling performances of the full cells of Example P1-E2；

Figure 15 shows the normalized energy densities of the full cells of Example P1-E2；

Figure 16 shows the cycling performances of the full cells of Example P1-E3 with the prelithiation degrees ε of a) 0 and b) 22％；

Figure 17 shows the discharge/charge curve of the cell of Comparative Example P2-CE1, wherein “1” , “4” , “50” and “100” stand for the 1^st, 4^th, 50^th and 100^th cycle respectively；

Figure 18 shows the discharge/charge curve of the cell of Example P2-E1, wherein “1” , “4” , “50” and “100” stand for the 1^st, 4^th, 50^th and 100^th cycle respectively；

Figure 19 shows the cycling performances of the cells of a) Comparative Example P2-CE1 (dashed line) and b) Example P2-E1 (solid line) ；

Figure 20 shows the average charge voltage a) and the average discharge voltage b) of the cell of Comparative Example P2-CE1；

Figure 21 shows the average charge voltage a) and the average discharge voltage b) of the cell of Example P2-E1.

Detailed Description of Preferred Embodiments

All publications, patent applications, patents and other references mentioned herein, if not otherwise indicated, are explicitly incorporated by reference herein in their entirety for all purposes as if fully set forth.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In case of conflict, the present specification, including definitions, will control.

When an amount, concentration, or other value or parameter is given as either a range, preferred range or a list of upper preferable values and lower preferable values, this is to be understood as specifically disclosing all ranges formed from any pair of any upper range limit or preferred value and any lower range limit or preferred value, regardless of whether ranges are separately disclosed. Where a range of numerical values is recited herein, unless otherwise stated, the range is intended to include the endpoints thereof, and all integers and fractions within the range.

The present invention relates to a method for producing silicon particles, said method including the following steps:

3) removing said heat absorbent and the oxidation products of said reducing agent；

wherein the melting temperature of said salt or the liquidus temperature of said composite salt ranges from a temperature higher than the heating temperature of step 2) to 800℃.

1) Preparing a mixture of a silica source material, a reducing agent, and a heat absorbent The way of preparing a mixture of a silica source material, a reducing agent, and a heat absorbent is not particularly limited. For example, the silica source material can be dispersed in an aqueous solution of the heat absorbent under stirring at room temperature. The mixture can be heated to 80℃ under vigorous stirring, dried under vacuum at 90℃ to remove water, and then homogenized by hand-milling in an agate mortar. Then the mixture and magnesium and/or aluminium powder can be ground together in an agate mortar.

In accordance with an embodiment of the method according to the present invention, the melting temperature of said salt or the liquidus temperature of said composite salt can be 660 –800℃, preferably 665 –790℃, more preferably 670 –780℃, for example 680℃, 690℃, 700℃, 710℃, 720℃, 730℃, 740℃, 750℃, 760℃, or 770℃. In addition to the melting temperature or liquidus temperature, the type of said salt or composite salt is not particularly limited. For example, said salt or composite salt shall not be decomposed under the heating temperature of step 2) , and preferably can be inorganic salts or composite salts, more preferably inorganic halides. Said salt or composite salt preferably does not contain water of crystallization, or is not susceptible to be hydrated.

In accordance with another embodiment of the method according to the present invention, said heat absorbent can be one or more selected from the group consisting of KCl；

KCl/LiCl with a LiCl content of ≤ 25 mol. ％, preferably ≤ 20 mol. ％, more preferably ≤ 10 mol. ％, particular preferably ≤ 5 mol. ％； and

KCl/NaCl with a NaCl content of ≤ 30 mol. ％or 66 –98 mol. ％, preferably ≤ 10 mol. ％or 85 –95 mol. ％.

In accordance with another embodiment of the method according to the present invention, the weight ratio of said silica source material to said heat absorbent can be 3 : 7 –7 : 3, preferably 2 : 3 –3 : 2, more preferably 4 : 5 –1 : 1, calculated on the basis of SiO₂ in said silica source material.

In accordance with another embodiment of the method according to the present invention, said silica source material can be one or more selected from the group consisting of zeolite, diatom, SiO₂ nanopowder, and porous SiO₂, preferably porous SiO₂, such as

350, available from EVONIK.

In accordance with another embodiment of the method according to the present invention, the amount of said reducing agent used can be 1 –1.5 times, preferably 1 –1 : 1.3 times, more preferably 1 –1.1 times the stoichiometry according to the reaction between SiO₂ and said reducing agent.

2) Heating the mixture

In accordance with another embodiment of the method according to the present invention, in step 2) the mixture obtained from step 1) can be heated at a heating temperature of at least 2℃, preferably 5℃, more preferably 10℃ higher than the melting point of said reducing agent, for 1 –6 hours, preferably 2 –3 hours, for example at a heating rate of 2℃/min, 5℃/min, or 10℃/min, under a protective atmosphere, for example Ar/H₂ (5 vol. ％) . The heating rate and the heating duration are not particularly limited. The furnace used here is not particularly limited. For examle an ordinary tube furnace can be used for heating the mixture obtained from step 1) . In case of large-scale production of silicon particles, it would be preferred to use a tube furnace rotatable along its longitudinal axis, namely a rotation furnace.

3) Removing said heat absorbent and the oxidation products of said reducing agent Firstly the heat absorbent can be removed by immersing the product of step 2) into water and filtering, and can also be recycled by drying the filtrate. And then the oxidation products of said reducing agent can be removed by immersing the filter residue into 2 M HCl solution and stirred for 12 hours.

In accordance with another embodiment of the method according to the present invention, after step 3) HF can be used to rinse the product obtained from step 3) , so as to remove unreacted SiO₂ and/or SiO₂ newly grown on the surface of the silicon particles during step 3) . Especially in case that the secondary particle size of the silicon particles obtained from step 2) is relatively small, for example less than 0.5 μm, so that the surface of the silicon particles might be oxidized again during step 3) , it would be preferred to use HF to rinse the product obtained from step 3) . For example, the product obtained from step 3) can be immersed into 1 wt. ％HF/EtOH (10 vol. ％) solution and stirred for 15 min. Finally, the product can be washed with distilled water and ethanol until pH ＝ 7 and then dried under vacuum at 65℃ for 10 hours. Otherwise, it would be not very necessary to use HF to rinse the product obtained from step 3) in case that the secondary particle size of the silicon particles obtained from step 2) is relatively large, for example greater than or equal to 0.5 μm.

The present invention further relates to porous silicon particles, which have a bimodal pore size distribution of < 2 nm and 10 –30 nm.

In accordance with an embodiment of the porous silicon particles according to the present invention, said porous silicon particles have a BET specific surface area of greater than 300 m²/g, preferably greater than 400 m²/g, more preferably greater than 500 m²/g.

The micropores of < 2 nm in the porous silicon particles are supposed to be derived from porous SiO₂ as the silica source material. In addition, the BET specific surface area of the porous silicon particles is one order of magnitude greater than that of porous SiO₂ as the silica source material. The inventors of the present invention believe that porous SiO₂ as the silica source material may contain quite a number of closed micropores, and these closed micropores may be opened during the vigorous reduction reaction and may contribute to the specific surface area.

In accordance with another embodiment of the porous silicon particles according to the present invention, the primary particle size of said porous silicon particles can be 30 –100 nm, preferably 35 –80 nm； and the secondary particle size (agglomerate particle size) of said porous silicon particles can be 1 –10 μm, prefaerably 3 –6 μm.

In accordance with another embodiment of the porous silicon particles according to the present invention, the pore volume of said porous silicon particles can be 0.1 –1.5 cm³/g. In accordance with another embodiment of the porous silicon particles according to the present invention, said porous silicon particles can be prepared by the method according to the present invention, in case that porous SiO₂ is used as the silica source material.

In accordance with an embodiment of the silicon-carbon composite according to the present invention, the thickness of the carbon coating layer can be 1 –10 nm.

In general, when the cathode efficiency is higher than the anode efficiency, a prelithiation can effectively increase the cell capacity via increasing the initial Coulombic efficiency. In this case, maximum energy density can be reached. For a cell, in which the loss of lithium during cycling may occur, prelithiation can also improve the cycling performance when an over-prelithiation is applied. The over-prelithiation provides a reservoir of lithium in the whole electrochemical system and the extra lithium in the anode compensates the possible lithium consumption from the cathode during cycling.

In principle, the higher prelithiation degree, the better cycling performance could be achieved. However, a higher prelithiation degree involves a much larger anode. Therefore, the cell energy density will decrease due to the increased weight and volume of the anode. Therefore, the prelithiation degree should be carefully controlled to balance the cycling performance and the energy density.

The present invention, according to one aspect, relates to a lithium-ion battery comprising a cathode, an electrolyte, and an anode, wherein the anode comprises the electrode material according to the present invention, and the initial surface capacity a of the cathode and the initial surface capacity b of the anode satisfy the relation formulae

1 < (b · (1 –ε) /a) ≤ 1.2 (I) ,

0 < ε ≤ ( (a·η₁) /0.6 – (a–b · (1 –η₂) ) ) /b (II) ,

where

ε is the prelithiation degree of the anode,

η₁ is the initial coulombic efficiency of the cathode, and

η₂ is the initial coulombic efficiency of the anode.

In the context of the present invention, the term “surface capacity” means the specific surface capacity in mAh/cm², the electrode capacity per unit of the electrode surface area. The term “initial capacity of the cathode” means the initial delithiation capacity of the cathode, and the term “initial capacity of the anode” means the initial lithiation capacity of the anode.

According to the present invention, the term “prelithiation degree” ε of the anode can be calculated by (b –a ·x) /b, wherein x is the balance of the anode capacity after prelithiation and the cathode capacity. For safety reasons, the anode capacity is usually designed slightly greater than the cathode capacity, and the balance of the anode capacity after prelithiation and the cathode capacity can be selected from greater than 1 to 1.2, preferably from 1.05 to 1.15, more preferably from 1.08 to 1.12, particular preferably about 1.1.

In accordance with an embodiment of the lithium-ion battery according to the present invention, the initial surface capacity a of the cathode and the initial surface capacity b of the anode satisfy the relation formulae

1.05 ≤ (b · (1 –ε) /a) ≤ 1.15 (Ia) ,

preferably 1.08 ≤ (b · (1 –ε) /a) ≤ 1.12 (Ib) .

In accordance with another embodiment of the lithium-ion battery according to the present invention, the prelithiation degree of the anode can be defined as

ε ＝ ( (a·η₁) /c – (a–b · (1 –η₂) ) ) /b (III) ,

0.6 ≤ c < 1 (IV) ,

preferably 0.7 ≤ c < 1 (IVa) ,

more preferably 0.7 ≤ c ≤ 0.9 (IVb) ,

particular preferably 0.75 ≤ c ≤ 0.85 (IVc) ,

where

c is the depth of discharge (DoD) of the anode.

In particular, ε ＝ (b · (1 –η₂) –a · (1 –η₁) ) /b, when c ＝ 1.

In accordance with another embodiment of the lithium-ion battery according to the present invention, the active material of the anode can be selected from the group consisting of carbon, silicon, silicon intermetallic compound, silicon oxide, silicon alloy and mixtures thereof.

In accordance with another embodiment of the lithium-ion battery according to the present invention, the active material of the cathode can be selected from the group consisting of lithium nickel oxide, lithium cobalt oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium nickel cobalt manganese oxide, and mixtures thereof.

The present invention, according to another aspect, relates to a method for producing a lithium-ion battery comprising a cathode, an electrolyte, and an anode, wherein the anode comprises the electrode material according to the present invention, and said method includes the following steps:

1) prelithiating the active material of the anode or the anode to a prelithiation degree ε, and

2) assembling the anode and the cathode to obtain said lithium-ion battery, characterized in that the initial surface capacity a of the cathode, the initial surface capacity b of the anode, and the prelithiation degree ε satisfy the relation formulae

1 < (b · (1 –ε) /a) ≤ 1.2 (I) ,

0 < ε ≤ ( (a·η₁) /0.6 – (a–b · (1 –η₂) ) ) /b (II) ,

where

ε is the prelithiation degree of the anode,

η₁ is the initial coulombic efficiency of the cathode, and

η₂ is the initial coulombic efficiency of the anode.

The prelithiation process is not particularly limited. The lithiation of the anode active material substrate can be carried out for example in several different ways. A physical process includes deposition of a lithium coating layer on the surface of the anode active material substrate such as silicon particles, thermally induced diffusion of lithium into the substrate such as silicon particles, or spray of stabilized Li powder onto the anode tape. An electrochemical process includes using silicon particles and a lithium metal plate as the electrodes, and applying an electrochemical potential so as to intercalate Li⁺ ions into the bulk of the silicon particles. An alternative electrochemical process includes assembling a half cell with silicon particles and Li metal foil electrodes, charging the half cell, and disassembling the half cell to obtain lithiated silicon particles.

In accordance with an embodiment of the method according to the present invention, the initial surface capacity a of the cathode and the initial surface capacity b of the anode satisfy the relation formulae

1.05 ≤ (b · (1 –ε) /a) ≤ 1.15 (Ia) ,

preferably 1.08 ≤ (b · (1 –ε) /a) ≤ 1.12 (Ib) .

In accordance with another embodiment of the method according to the present invention, the prelithiation degree of the anode can be defined as

ε ＝ ( (a·η₁) /c – (a–b · (1 –η₂) ) ) /b (III) ,

0.6 ≤ c < 1 (IV) ,

preferably 0.7 ≤ c < 1 (IVa) ,

more preferably 0.7 ≤ c ≤ 0.9 (IVb) ,

particular preferably 0.75 ≤ c ≤ 0.85 (IVc) ,

where

c is the depth of discharge (DoD) of the anode.

In accordance with another embodiment of the method according to the present invention, the active material of the anode can be selected from the group consisting of carbon, silicon, silicon intermetallic compound, silicon oxide, silicon alloy and mixtures thereof.

In accordance with another embodiment of the method according to the present invention, the active material of the cathode can be selected from the group consisting of lithium nickel oxide, lithium cobalt oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium nickel cobalt manganese oxide, and mixtures thereof.

Prior art prelithiation methods often involve a treatment of coated anode tape. This could be an electrochemical process, or physical contact of the anode with stabilized lithium metal powder. However, these prelithiation procedure requires additional steps to the current battery production method. Furthermore, due to the highly active nature of the prelithiated anode, the subsequent battery production procedure requires an environment with well-controlled humidity, which results in an increased cost for the cell production.

The present invention provides an alternative method of in-situ prelithiation. The lithium source for prelithaition comes from the cathode. During the first formation cycle, by increasing the cut-off voltage of the full cell, additional amount of lithium is extracted from the cathode； by controlling the discharge capacity, the additional lithium extracted from the cathode is stored at the anode, and this is ensured in the following cycles by maintaining the upper cut-off voltage the same as in the first cycle.

The present invention, according to another aspect, relates to a lithium-ion battery comprising a cathode, an electrolyte, and an anode, characterized in that the anode comprises the electrode material according to the present invention, and said lithium-ion battery is subjected to a formation process, wherein said formation process includes an initial formation cycle comprising the following steps:

a) charging the battery to a cut off voltage V_off which is greater than the nominal charge cut off voltage of the battery, and

b) discharging the battery to the nominal discharge cut off voltage of the battery.

In the context of the present invention, the term “formation process” means the initial one or more charging/discharging cycles of the lithium-ion battery for example at 0.1C, once the lithium-ion battery is assembled. During this process, a stable solid-electrolyte-inter-phase (SEI) layer can be formed at the anode.

In accordance with an embodiment of the formation process according to the present invention, in step a) the battery can be charged to a cut off voltage which is up to 0.8 V greater than the nominal charge cut off voltage of the battery, preferably 0.1 ～ 0.5 V greater than the nominal charge cut off voltage of the battery, more preferably 0.2 ～ 0.4 V greater than the nominal charge cut off voltage of the battery, particular preferably about 0.3 V greater than the nominal charge cut off voltage of the battery.

A lithium-ion battery with the typical cathode materials of cobalt, nickel, manganese and aluminum typically charges to 4.20V ± 50mV as the nominal charge cut off voltage. Some nickel-based batteries charge to 4.10V ± 50mV.

In accordance with another embodiment of the formation process according to the present invention, the nominal charge cut off voltage of the battery can be about 4.2 V ± 50mV, and the nominal discharge cut off voltage of the battery can be about 2.5 V ± 50mV.

In accordance with another embodiment of the formation process according to the present invention, the Coulombic efficiency of the cathode in the initial formation cycle can be 40％～ 80％, preferably 50％～ 70％.

In accordance with another embodiment of the formation process according to the present invention, said formation process further includes one or two or more formation cycles, which are carried out in the same way as the initial formation cycle.

For the traditional lithium-ion batteries, when the battery is charged to a cut off voltage greater than the nominal charge cut off voltage, metallic lithium will be plated on the anode, the cathode material becomes an oxidizing agent, produces carbon dioxide (CO₂) , and increases the battery pressure.

In case of a preferred lithium-ion battery defined below according to the present invention, when the battery is charged to a cut off voltage greater than the nominal charge cut off voltage, additional Li⁺ ions can be intercalated into the anode having additional capacity, instead of being plated on the anode.

In case of another preferred lithium-ion battery defined below according to the present invention, in which the electrolyte comprises one or more fluorinated carbonate compounds as a nonaqueous organic solvent, the electrochemical window of the electrolyte can be broadened, and the safety of the battery can still be ensured at a charge cut off voltage of 5V or even higher.

In order to implement the present invention, an additional cathode capacity can preferably be supplemented to the nominal initial surface capacity of the cathode.

In the context of the present invention, the term “nominal initial surface capacity” a of the cathode means the nominally designed initial surface capacity of the cathode.

In accordance with an embodiment of the lithium-ion battery according to the present invention, the relative increment r of the initial surface capacity of the cathode over the nominal initial surface capacity a of the cathode and the cut off voltage V_off satisfy the following linear equation with a tolerance of ±5％, ±10％, or ±20％

r ＝ 0.75V_off –3.134 (V) .

In accordance with another embodiment of the lithium-ion battery according to the present invention, the relative increment r of the initial surface capacity of the cathode over the nominal initial surface capacity a of the cathode and the cut off voltage V_off satisfy the following quadratic equation with a tolerance of ±5％, ±10％, or ±20％

r ＝ –0.7857V_off ² + 7.6643V_off –18.33 (Va) .

In accordance with another embodiment of the lithium-ion battery according to the present invention, the nominal initial surface capacity a of the cathode and the initial surface capacity b of the anode satisfy the relation formulae

1 < b ·η₂ / (a· (1 + r) –b · (1 –η₂) ) –ε ≤ 1.2 (I′) ,

preferably 1.05 ≤ b ·η₂ / (a· (1 + r) –b · (1 –η₂) ) –ε ≤ 1.15 (Ia′) ,

more preferably 1.08 ≤ b ·η₂ / (a· (1 + r) –b · (1 –η₂) ) –ε ≤ 1.12 (Ib′) ,

0 < ε ≤ ( (a·η₁) /0.6 – (a–b · (1 –η₂) ) ) /b (II) ,

where

ε is the prelithiation degree of the anode, and

η₂ is the initial coulombic efficiency of the anode.

ε ＝ ( (a·η₁) /c – (a–b · (1 –η₂) ) ) /b (III) ,

0.6 ≤ c < 1 (IV) ,

preferably 0.7 ≤ c < 1 (IVa) ,

more preferably 0.7 ≤ c ≤ 0.9 (IVb) ,

particular preferably 0.75 ≤ c ≤ 0.85 (IVc) ,

where

η₁ is the initial coulombic efficiency of the cathode, and

c is the depth of discharge (DoD) of the anode.

In accordance with another embodiment of the lithium-ion battery according to the present invention, the electrolyte comprises one or more fluorinated carbonate compounds, preferably fluorinated cyclic or acyclic carbonate compounds, as a nonaqueous organic solvent.

In accordance with another embodiment of the lithium-ion battery according to the present invention, the fluorinated carbonate compounds can be selected from the group consisting of fluorinated ethylene carbonate, fluorinated propylene carbonate, fluorinated dimethyl carbonate, fluorinated methyl ethyl carbonate, and fluorinated diethyl carbonate, in which the “fluorinated” carbonate compounds can be understood as “monofluorinated” , “difluorinated” , “trifluorinated” , “tetrafluorinated” , and “perfluorinated” carbonate compounds.

In accordance with another embodiment of the lithium-ion battery according to the present invention, the fluorinated carbonate compounds can be selected from the group consisting of monofluoroethylene carbonate, 4, 4-difluoro ethylene carbonate, 4, 5-difluoro ethylene carbonate, 4, 4, 5-trifluoroethylene carbonate, 4, 4, 5, 5-tetrafluoroethylene carbonate, 4-fluoro-4-methyl ethylene carbonate, 4, 5-difluoro-4-methyl ethylene carbonate, 4-fluoro-5-methyl ethylene carbonate, 4, 4-difluoro-5-methyl ethylene carbonate, 4- (fluoromethyl) -ethylene carbonate, 4- (difluoromethyl) -ethylene carbonate, 4- (trifluoromethyl) -ethylene carbonate, 4- (fluoromethyl) -4-fluoro ethylene carbonate, 4- (fluoromethyl) -5-fluoro ethylene carbonate, 4, 4, 5-trifluoro-5-methyl ethylene carbonate, 4-fluoro-4, 5-dimethyl ethylene carbonate, 4, 5-difluoro-4, 5-dimethyl ethylene carbonate, and 4, 4-difluoro-5, 5-dimethyl ethylene carbonate.

In accordance with another embodiment of the lithium-ion battery according to the present invention, the content of the fluorinated carbonate compounds can be 10 ～ 100 vol. ％, preferably 30 ～ 100 vol. ％, more preferably 50 ～ 100 vol. ％, particular preferably 80 ～ 100 vol. ％, based on the total nonaqueous organic solvent.

In accordance with another embodiment of the lithium-ion battery according to the present invention, after being subjected to the formation process, said lithium-ion battery can still be charged to a cut off voltage V_off, which is greater than the nominal charge cut off voltage of the battery, and be discharged to the nominal discharge cut off voltage of the battery.

In accordance with another embodiment of the lithium-ion battery according to the present invention, after being subjected to the formation process, said lithium-ion battery can still be charged to a cut off voltage V_off, which is up to 0.8 V greater than the nominal charge cut off voltage of the battery, more preferably 0.1 ～ 0.5 V greater than the nominal charge cut off voltage of the battery, particular preferably 0.2 ～ 0.4 V greater than the nominal charge cut off voltage of the battery, especially preferably about 0.3 V greater than the nominal charge cut off voltage of the battery, and be discharged to the nominal discharge cut off voltage of the battery.

1) assembling the anode and the cathode to obtain said lithium-ion battery, and

2) subjecting said lithium-ion battery to a formation process, wherein said formation process includes an initial formation cycle comprising the following steps:

In accordance with an embodiment of the method according to the present invention, the relative increment r of the initial surface capacity of the cathode over the nominal initial surface capacity a of the cathode and the cut off voltage V_off satisfy the following linear equation with a tolerance of ±5％, ±10％, or ±20％

r ＝ 0.75V_off –3.134 (V) .

In accordance with another embodiment of the method according to the present invention, the relative increment r of the initial surface capacity of the cathode over the nominal initial surface capacity a of the cathode and the cut off voltage V_off satisfy the following quadratic equation with a tolerance of ±5％, ±10％, or ±20％

r ＝ –0.7857V_off ² + 7.6643V_off –18.33 (Va) .

In accordance with another embodiment of the method according to the present invention, the nominal initial surface capacity a of the cathode and the initial surface capacity b of the anode satisfy the relation formulae

1 < b ·η₂ / (a· (1 + r) –b · (1 –η₂) ) –ε ≤ 1.2 (I′) ,

0 < ε ≤ ( (a·η₁) /0.6 – (a–b · (1 –η₂) ) ) /b (II) ,

where

ε is the prelithiation degree of the anode, and

η₂ is the initial coulombic efficiency of the anode.

ε ＝ ( (a·η₁) /c – (a–b · (1 –η₂) ) ) /b (III) ,

0.6 ≤ c < 1 (IV) ,

preferably 0.7 ≤ c < 1 (IVa) ,

more preferably 0.7 ≤ c ≤ 0.9 (IVb) ,

particular preferably 0.75 ≤ c ≤ 0.85 (IVc) ,

where

η₁ is the initial coulombic efficiency of the cathode, and

c is the depth of discharge (DoD) of the anode.

In accordance with another embodiment of the method according to the present invention, the electrolyte comprises one or more fluorinated carbonate compounds, preferably fluorinated cyclic or acyclic carbonate compounds, as a nonaqueous organic solvent.

In accordance with another embodiment of the method according to the present invention, the fluorinated carbonate compounds can be selected from the group consisting of fluorinated ethylene carbonate, fluorinated propylene carbonate, fluorinated dimethyl carbonate, fluorinated methyl ethyl carbonate, and fluorinated diethyl carbonate, in which the “fluorinated” carbonate compounds can be understood as “monofluorinated” , “difluorinated” , “trifluorinated” , “tetrafluorinated” , and “perfluorinated” carbonate compounds.

In accordance with another embodiment of the method according to the present invention, the fluorinated carbonate compounds can be selected from the group consisting of monofluoroethylene carbonate, 4, 4-difluoro ethylene carbonate, 4, 5-difluoro ethylene carbonate, 4, 4, 5-trifluoroethylene carbonate, 4, 4, 5, 5-tetrafluoroethylene carbonate, 4-fluoro-4-methyl ethylene carbonate, 4, 5-difluoro-4-methyl ethylene carbonate, 4-fluoro-5-methyl ethylene carbonate, 4, 4-difluoro-5-methyl ethylene carbonate, 4- (fluoromethyl) -ethylene carbonate, 4- (difluoromethyl) -ethylene carbonate, 4- (trifluoromethyl) -ethylene carbonate, 4- (fluoromethyl) -4-fluoro ethylene carbonate, 4- (fluoromethyl) -5-fluoro ethylene carbonate, 4, 4, 5-trifluoro-5-methyl ethylene carbonate, 4-fluoro-4, 5-dimethyl ethylene carbonate, 4, 5-difluoro-4, 5-dimethyl ethylene carbonate, and 4, 4-difluoro-5, 5-dimethyl ethylene carbonate.

In accordance with another embodiment of the method according to the present invention, the content of the fluorinated carbonate compounds can be 10 ～ 100 vol. ％, preferably 30 ～ 100 vol. ％, more preferably 50 ～ 100 vol. ％, particular preferably 80 ～ 100 vol. ％, based on the total nonaqueous organic solvent.

Example 1 (E1) :

KCl (melting temperature: 771℃) was used as the heat absorbent. In particular, 0.5 g of nano-SiO₂ power (Aladdin Chemical, 15 nm) was firstly dispersed to an aqueous KCl solution (0.1 g/mL) under stirring at room temperature. The ratio of silica to KCl in weight was 30: 70. The mixture was heated to 80℃ under vigorous stirring followed by drying under vacuum at 90℃ to remove water. Dried nano-SiO₂/KCl powder was then homogenized by hand-milling in an agate mortar.

A mixture of above nano-SiO₂/KCl powder and magnesium powder (Sinopharm Chemical Reagent Co. Ltd, 100～200 mesh) was ground together in the agate mortar at a molar ratio of Mg/SiO₂＝2.0. Next, the obtained mixture was loaded in an alundum boat and placed in the constant temperature zone of a tube furnace. And then the furnace was heated from room temperature to 650℃ at a rate of 2℃/min and kept at 650℃ for 4 hours under Ar (95 vol. ％) /H₂ (5 vol. ％) mixed atmosphere. Finally, after cooling to room temperature, a uniform powder in yellow color was obtained.

The obtained product after magnesiothermic reduction was firstly immersed in water and filtered, where KCl can be recycled by drying the filtrate. And then the residue was immersed into 2 M HCl solution and stirred for 12 hours to remove MgO. To further remove small amount of unreacted and surface-grown SiO₂, 1 wt. ％HF/EtOH (10 vol. ％) solution was used and stirred for 15 min. Finally, silicon products were washed with distilled water and ethanol until pH ＝ 7 and then vacuum dried at 65℃ for 10 hours.

Structural evaluation:

X-Ray Diffraction (XRD) was used to analyse the composition, the crystallinity and the crystal size of the products. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) were employed to characterize the size and structure of the products. N₂-sorption isotherms were used to analyse the pore size distribution of the products.

Figure 1 shows the XRD pattern of the silicon particles of Example 1. It can be seen that there was no impurity in the silicon particles of Example 1.

Cells assembling and electrochemical evaluation:

The electrochemical performances of the as-prepared composites were tested using two-electrode coin-type cells. The working electrodes were prepared by pasting a mixture of active material, Super P conductive carbon black (40 nm, Timical) and styrene butadiene rubber/sodium carboxymethyl cellulose (SBR/SCMC, 3: 5 by weight) as binder at a weight ratio of 60: 20: 20. After coating the mixture onto pure Cu foil, the electrodes were dried, cut to Φ12 mm sheets, and then further dried at 60℃ in vacuum for 4 hours. The CR2016 coin cells were assembled in an argon-filled glove box (MB-10 compact, MBraun) using 1 M LiPF₆/EC+DMC (1: 1 by volume, ethylene carbonate (EC) , dimethyl carbonate (DMC)) as electrolyte, including 10％Fluoroethylene carbonate (FEC) , ENTEK ET20-26 as separator, and pure lithium foil as counter electrode. The cycling performances were evaluated on a LAND battery test system (Wuhan Kingnuo Electronics Co., Ltd., China) at 25℃, wherein in the cycling performance test the coin cells were discharged at 100 mA g^-1 for the initial two cycles and at 300 mA g^-1 (Examples 1 ～ 7 and Comparison Examples 1 ～ 3) or 1000 mA g^-1 (Examples 8 ～ 9) for the following cycles. The cut-off voltage was 0.01 V versus Li/Li⁺ for discharge (Li insertion) and 1.2 V versus Li/Li⁺ for charge (Li extraction) .

Figures 2 and 3 show the cycling performance of the silicon particles of Example 1.

Comparative Example 1 (CE1) :

Comparative Example 1 was carried out similar to Example 1, except that no KCl was used as the heat absorbent.

Figures 2 and 3 show the cycling performance of the silicon particles of Comparison Example 1. It can be seen that the electrochemical performance of the silicon particles can be greatly enhanced by using a salt or composite salt as the heat absorbent.

Example 2 (E2) :

Example 2 was carried out similar to Example 1, except that 0.54 g of nano-SiO₂ power was used, and the weight ratio of silica to the heat absorbent was 45: 55.

Figure 3 shows the cycling performance of the silicon particles of Example 2. Figure 4 shows the XRD patterns of the silicon particles of Example 2. Figure 5 shows the SEM images of the silicon particles of Example 2.

Comparative Example 2 (CE2) :

Comparative Example 2 was carried out similar to Example 2, except that NaCl (melting temperature: 801℃) was used as the heat absorbent.

Figure 3 shows the cycling performance of the silicon particles of Comparative Example 2.

Figure 4 shows the XRD patterns of the silicon particles of Comparison Example 2. Figure 5 shows the SEM images of the silicon particles of Comparison Example 2.

It can be seen that the particle size of the silicon particles of Comparison Example 2 was too small, and the crystallinity and the crystal size of the silicon particles of Comparison Example 2 were relatively low.

The inventors of the present invention believed that the surface of the silicon particles of Comparison Example 2 was too active and might be oxidized before being used as the electrode material, even though HF was used to rinse the product.

Example 3 (E3) :

Example 3 was carried out similar to Example 2, except that KCl/NaCl with a NaCl content of 10 mol. ％was used as the heat absorbent, the liquidus temperature of which can be determined from the binary diagram of KCl/NaCl as about 720℃.

Figure 3 shows the cycling performance of the silicon particles of Example 3. Figure 4 shows the XRD patterns of the silicon particles of Example 3. Figure 5 shows the SEM images of the silicon particles of Example 3.

Example 4 (E4) :

Example 4 was carried out similar to Example 2, except that KCl/NaCl with a NaCl content of 72 mol. ％was used as the heat absorbent, the liquidus temperature of which can be determined from the binary diagram of KCl/NaCl as about 670℃.

Figure 3 shows the cycling performance of the silicon particles of Example 4.

Example 5 (E5) :

Example 5 was carried out similar to Example 2, except that KCl/NaCl with a NaCl content of 90 mol. ％was used as the heat absorbent, the liquidus temperature of which can be determined from the binary diagram of KCl/NaCl as about 715℃.

Figure 3 shows the cycling performance of the silicon particles of Example 5. Figure 4 shows the XRD patterns of the silicon particles of Example 5. Figure 5 shows the SEM images of the silicon particles of Example 5.

Example 6 (E6) :

Example 6 was carried out similar to Example 2, except that KCl/LiCl with a LiCl content of 5 mol. ％was used as the heat absorbent, the liquidus temperature of which can be determined from the binary diagram of KCl/LiCl as about 750℃.

Figure 3 shows the cycling performance of the silicon particles of Example 6. Figure 4 shows the XRD patterns of the silicon particles of Example 6.

The crystal size of the silicon particles can be calculated based on the XRD patterns according to Scherrer formula. The sequence of the crystal size of the silicon particles were: CE2 < E2 ≈ E5 < E6 < E3.

Figure 5 shows the SEM images of the silicon particles of Example 6. The particle size can be measured based on the SEM images of the silicon particles.

Table 1

	E2	E3	E5	E6	CE2
Reversible Capacity (mAh/g)	2101	2951	2819	2716	1809
1^st Coulombic Efficiency	67.5％	86.2％	71.0％	79.0％	56.0％
Particle Size (nm)	30	100	35	70	10

Example 7 (E7) :

Example 7 was carried out similar to Example 2, except that 3.83 g of nano-SiO₂ power was used and a rotation furnace was used instead of the tube furnace.

Figure 3 shows the cycling performance of the silicon particles of Example 7. Figure 6 shows the XRD patterns of the silicon particles of Example 7.

Comparative Example 3 (CE3) :

Comparative Example 3 was carried out similar to Example 1, except that 2 g of nano-SiO₂ power was used, NaCl (melting temperature: 801℃) was used as the heat absorbent and the weight ratio of silica to the heat absorbent was 1: 10. Such a weight ratio was used according to Luo, W. ’s synthesis method. The high content of NaCl resulted in a low capacity.

Figure 3 shows the cycling performance of the silicon particles of Comparative Example 3.

Figure 6 shows the XRD patterns of the silicon particles of Comparison Example 3. It can be seen that the product of Comparative Example 3 contained SiO₂ impurity. The peaks at 69° and 76° were too weak, which demonstrated that the crystallinity of the product of Comparative Example 3 was relatively low. In addition, the FWHM of the product of Comparative Example 3 was broader than that of Example 7, which demonstrated that the crystal size of the product of Comparative Example 3 was smaller than that of Example 7.

The inventors of the present invention believed that the surface of the silicon particles of Comparison Example 3 was too active and might be oxidized before being used as the electrode material, even though HF was used to rinse the product.

Example 8 (E8) :

Example 8 was carried out similar to Example 7, except that porous SiO₂ (

350, available from EVONIK) was used as the silica source material to obtain porous silicon particles as the product and that HF was not used to rinse the product.

350 is a macroporous silica with low surface area and an average pore size in the 150 nm range. Its specific surface area (N₂, multipoint, following ISO 9277) is 55 m²/g. Its particle size (d50, laser diffraction, following ISO 13320-1) is 4.5 μm.

Figure 7 shows (a) the SEM and (b) the TEM images of the silicon particles of Example 8.

Figure 8 shows the N₂-sorption isotherms of (a) the raw material porous SiO₂ and (b) the silicon particles of Example 8. Figure 9 shows the pore size distribution of (a) the raw material porous SiO₂ and (b) the silicon particles of Example 8. It can be seen that the porous silicon particles of Example 8 have a bimodal pore size distribution of < 2 nm and 10 –30 nm.Figure 10 shows the cycling performances of the silicon particles of Examples 8.

Example 9 (E9) :

A carbon coating was applied on the porous silicon particles obtained from Example 8 by CVD, and the carbon content was 26 wt. ％and the carbon layer thickness is ca. 6nm.

Figure 10 shows the cycling performances of the silicon-carbon composite of Example 9. Figure 11 shows the rate capability of the silicon-carbon composite of Example 9.

EXAMPLES P1 for Prelithiation

Active material of the cathode: NCM-111 from BASF, and HE-NCM prepared according to the method as described in WO 2013/097186 A1；

Active material of the anode: a mixture (1: 1 by weight) of silicon nanoparticle with a diameter of 50 nm from Alfa Aesar and graphite from Shenzhen Kejingstar Technology Ltd. ；

Carbon additives: flake graphite KS6L and Super P Carbon Black C65 from Timcal；

Binder: PAA, Mv＝ 450, 000, from Sigma Aldrich；

Electrolyte: 1M LiPF₆/EC (ethylene carbonate) +DMC (dimethyl carbonate) (1: 1 by volume) ； Separator: PP/PE/PP membrane Celgard 2325.

Example P1-E1:

At first anode/Li half cells were assembled in form of 2016 coin cell in an Argon-filled glove box (MB-10 compact, MBraun) , wherein lithium metal was used as the counter electrode. The assembled anode/Li half cells were discharged to the designed prelithiation degree ε as given in Table P1-E1, so as to put a certain amout of Li⁺ ions in the anode, i.e., the prelithiation of the anode. Then the half cells were disassembled. The prelithiated anode and NCM-111 cathode were assembled to obtain 2032 coin full cells. The cycling performances of the full cells were evaluated at 25℃ on an Arbin battery test system at 0.1C for formation and at 1C for cycling.

Table P1-E1

Group	a	η₁	b	η₂	ε	c	x	η_F	Life
G0	2.30	90％	2.49	87％	0	1.00	1.08	83％	339
G1	2.30	90％	2.68	87％	5.6％	0.99	1.10	86％	353
G2	2.30	90％	3.14	87％	19.5％	0.83	1.10	89％	616
G3	2.30	90％	3.34	87％	24.3％	0.77	1.10	88％	904
G4	2.30	90％	3.86	87％	34.6％	0.66	1.10	89％	1500

a initial delithiation capacity of the cathode [mAh/cm²] ；

η₁ initial Coulombic efficency of the cathode；

b initial lithiation capacity of the anode [mAh/cm²] ；

η₂ initial Coulombic efficency of the anode；

ε prelithiation degree of the anode；

c depth of discharge of the anode；

x ＝ b · (1 –ε) /a, balance of the anode and cathode capacities after prelithiation；

η_F initial Coulombic efficiency of the full cell；

Life cycle life of the full cell (80％capacity retention) .

Figure 12 shows the cycling performances of the full cells of Groups G0, G1, G2, G3, and G4 of Example P1-E1.

In case of Group G0 with a prelithiation degree ε ＝ 0, the capacity of the full cell was decreased to 80％after 339 cycles.

In case of Group G1 with a prelithiation degree of 5.6％, the prelithiation amount was only enough to compensate the irreversible Li loss difference between the cathode and the anode. Therefore, the initial Coulombic efficiency was increased from 83％to 86％, while no obvious improvement in cycling performance was observed.

In case of Group G2 with a prelithiation degree increased to 19.5％, the prelithiation amount was not only enough to compensate the irreversible Li loss difference between the cathode and the anode, but also extra amount of Li was reserved in the anode to compensate the Li loss during cycling. Hence, the cycle life was greatly improved to 616 cycles.

In case of Groups G3 and G4 with further increased prelithiation degrees, more and more Li was reserved in the anode, so better and better cycling performances were obtained.

Figure 13 shows a) the volumetric energy densities and b) the gravimetric energy densities of the full cells of Groups G0, G1, G2, G3, and G4 in Example P1-E1. Compared with non-prelithiation (G0) , Group G1 with 5.6％prelithiation degree shows a higher energy density due to the higher capacity. In case of the further increased prelithiation degree for a better cycling performance, the energy density decreases to some extend but still has more than 90％energy density of G0 when prelithiation degree reaches 34.6％in G4.

Example P1-E2:

Example P1-E2 was carried out similar to Example P1-E1, except that HE-NCM was used as the cathode active material and the corresponding parameters were given in Table P1-E2.

Table P1-E2

Group	a	η₁	b	η₂	ε	c	x	η_F	Life
G0	3.04	96％	3.25	87％	0	1.00	1.07	85％	136
G1	3.04	96％	4.09	87％	18.3％	0.90	1.10	94％	231
G2	3.04	96％	4.46	87％	26.3％	0.80	1.08	95％	316

a initial delithiation capacity of the cathode [mAh/cm²] ；

η₁ initial Coulombic efficency of the cathode；

b initial lithiation capacity of the anode [mAh/cm²] ；

η₂ initial Coulombic efficency of the anode；

ε prelithiation degree of the anode；

c depth of discharge of the anode；

η_F initial Coulombic efficiency of the full cell；

Life cycle life of the full cell (80％capacity retention) .

Figure 14 shows the cycling performances of the full cells of Groups G0, G1, and G2 of Example P1-E2. Figure 15 shows a) the volumetric energy densities and b) the gravimetric energy densities of the full cells of Groups G0, G1, and G2 of Example P1-E2. It can been seen from Table P1-E2 that the initial Coulombic efficiencies of the full cells were increased from 85％to 95％in case of the prelithiation. Although larger anodes were used for prelithiation, the energy density did not decrease, or even a higher energy density was reached, compared with non-prelithiation in G0. Moreover, the cycling performances were greatly improved, because the Li loss during cycling was compensated by the reserved Li.

Example P1-E3:

Example P1-E3 was carried out similar to Example P1-E1, except that pouch cells were assembled instead of coin cells, and the corresponding prelithiation degrees ε of the anode were a) 0 and b) 22％.

Figure 16 shows the cycling performances of the full cells of Example P1-E3 with the prelithiation degrees ε of a) 0 and b) 22％. It can been seen that the cycling performance was much improved in case of the prelithiation.

EXAMPLES P2 for Prelithiation

Size of the pouch cell: 46 mm × 68 mm (cathode) ； 48 mm × 71 mm (anode) ；

Cathode: 96.5 wt. ％of NCM-111 from BASF, 2 wt. ％of PVDF Solef 5130 from Sovey, 1 wt. ％of Super P Carbon Black C65 from Timcal, 0.5 wt. ％of conductive graphite KS6L from Timcal；

Anode: 40 wt. ％of Silicon from Alfa Aesar, 40 wt. ％of graphite from BTR, 10 wt. ％of NaPAA, 8 wt. ％of conductive graphite KS6L from Timcal, 2 wt. ％of Super P Carbon Black C65 from Timcal；

Electrolyte: 1M LiPF₆/EC+DMC (1: 1 by volume, ethylene carbonate (EC) , dimethyl carbonate (DMC) , including 30 vol. ％of fluoroethylene carbonate (FEC) , based on the total nonaqueous organic solvent) ；

Separator: PP/PE/PP membrane Celgard 2325.

Comparative Example P2-CE1:

A pouch cell was assembled with a cathode initial capacity of 3.83 mAh/cm² and an anode initial capacity of 4.36 mAh/cm² in an Argon-filled glove box (MB-10 compact, MBraun) . The cycling performance was evaluated at 25℃ on an Arbin battery test system at 0.1C for formation and at 1C for cycling, wherein the cell was charged to the nominal charge cut off voltage 4.2 V, and discharged to the nominal discharge cut off voltage 2.5 V or to a cut off capacity of 3.1 mAh/cm². The calculated prelithiation degree ε of the anode was 0.

Figure 17 shows the discharge/charge curve of the cell of Comparative Example P2-CE1, wherein “1” , “4” , “50” and “100” stand for the 1^st, 4^th, 50^th and 100^th cycle respectively. Figure 19 shows the cycling performances of the cells of a) Comparative Example P2-CE1 (dashed line) . Figure 20 shows the average charge voltage a) and the average discharge voltage b) of the cell of Comparative Example P2-CE1.

Example P2-E1:

A pouch cell was assembled with a cathode initial capacity of 3.73 mAh/cm² and an anode initial capacity of 5.17 mAh/cm² in an Argon-filled glove box (MB-10 compact, MBraun) . The cycling performance was evaluated at 25℃ on an Arbin battery test system at 0.1C for formation and at 1C for cycling, wherein the cell was charged to a cut off voltage of 4.5 V, which was 0.3 V greater than the nominal charge cut off voltage, and discharged to the nominal discharge cut off voltage 2.5 V or to a cut off capacity of 3.1 mAh/cm². The calculated prelithiation degree ε of the anode was 21％.

Figure 18 shows the discharge/charge curve of the cell of Example P2-E1, wherein “1” , “4” , “50” and “100” stand for the 1^st, 4^th, 50^th and 100^th cycle respectively. Figure 19 shows the cycling performances of the cells of b) Example P2-E1 (solid line) . Figure 21 shows the average charge voltage a) and the average discharge voltage b) of the cell of Example P2-E1.

While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. The attached claims and their equivalents are intended to cover all the modifications, substitutions and changes as would fall within the scope and spirit of the invention.

Claims

A method for producing silicon particles, said method including the following steps:

1) preparing a mixture of a silica source material, magnesium and/or aluminium powder as a reducing agent, and a salt or composite salt as a heat absorbent；

2) heating the mixture obtained from step 1) at a heating temperature of from the melting point of said reducing agent to lower than 800℃ under a protective atmosphere；

3) removing said heat absorbent and the oxidation products of said reducing agent； characterized in that the melting temperature of said salt or the liquidus temperature of said composite salt ranges from a temperature higher than the heating temperature of step 2) to 800℃.
The method of claim 1, characterized in that the melting temperature of said salt or the liquidus temperature of said composite salt is 660–800℃, preferably 665–790℃, more preferably 670–780℃.
The method of claim 1 or 2, characterized in that the weight ratio of said silica source material to said heat absorbent is 3 : 7–7 : 3, preferably 2 : 3–3 : 2, more preferably 4 :5–1 : 1, calculated on the basis of SiO₂ in said silica source material.
The method of any one of claims 1 to 3, characterized in that said silica source material is one or more selected from the group consisting of zeolite, diatom, SiO₂ nanopowder, and porous SiO₂.
The method of any one of claims 1 to 4, characterized in that said heat absorbent is one or more selected from the group consisting of KCl； KCl/LiCl with a LiCl content of ≤ 25 mol. ％, preferably ≤ 20 mol. ％, more preferably ≤ 10 mol. ％； and KCl/NaCl with a NaCl content of ≤ 30 mol. ％ or 66–98 mol. ％, preferably ≤ 10 mol. ％ or 85–95 mol. ％.
The method of any one of claims 1 to 5, characterized in that the amount of said reducing agent used is 1–1.5 times the stoichiometry according to the reaction between SiO₂ and said reducing agent, preferably 1–1 : 1.3 times, more preferably 1–1.1 times.
The method of any one of claims 1 to 6, characterized in that in step 2) the mixture obtained from step 1) is heated at a heating temperature of at least 2℃, preferably 5℃, more preferably 10℃ higher than the melting point of said reducing agent, for 1–6 hours, preferably 2–3 hours.
The method of any one of claims 1 to 7, characterized in that after step 3) HF is used to rinse the product obtained from step 3) .
Porous silicon particles, characterized in that said porous silicon particles have a bimodal pore size distribution of < 2 nm and 10–30 nm.
Porous silicon particles of claim 9, characterized in that said porous silicon particles have a BET specific surface area of greater than 300 m²/g, preferably greater than 400 m²/g, more preferably greater than 500 m²/g.
Porous silicon particles of claim 9 or 10, characterized in that the primary particle size of said porous silicon particles is 30–100 nm, preferably 35–80 nm； and the secondary particle size of said porous silicon particles is 1–10 μm, prefaerably 3–6 μm.
Porous silicon particles of any one of claims 9 to 11, characterized in that the pore volume of said porous silicon particles is 0.1–1.5 cm³/g.
Porous silicon particles of any one of claims 9 to 12, characterized in that said porous silicon particles are prepared by the method of any one of claims 1 to 8, and the silica source material is porous SiO₂.
A silicon-carbon composite, characterized in that said silicon-carbon composite comprises a carbon coating layer as well as the porous silicon particles of any one of claims 9 to 13 or the silicon particles prepared by the method of any one of claims 1 to 8.
The silicon-carbon composite of claim 14, characterized in that the thickness of the carbon coating layer is 1–10 nm.
An electrode material, characterized in that said electrode material comprises the silicon-carbon composite of claim 14 or 15.
A battery, characterized in that said battery comprises the electrode material of claim 16.
The use of the silicon-carbon composite of claim 14 or 15 as an electrode active material.
A lithium-ion battery comprising a cathode, an electrolyte, and an anode, wherein the anode comprises the electrode material of claim 16, and the initial surface capacity a of the cathode and the initial surface capacity b of the anode satisfy the relation formulae

                                 1 < (b · (1–ε) /a) ≤ 1.2                     (I) ,

preferably                      1.05 ≤ (b · (1–ε) /a) ≤ 1.15                (Ia) ,

more preferably                 1.08 ≤ (b · (1–ε) /a) ≤ 1.12                (Ib) ,

                    0 < ε ≤ ( (a · η₁) /0.6– (a–b · (1–η₂) ) ) /b        (II) ,

where

ε is the prelithiation degree of the anode,

η₁ is the initial coulombic efficiency of the cathode, and

η₂ is the initial coulombic efficiency of the anode.
The lithium-ion battery of claim 19, characterized in that

                 ε＝ ( (a · η₁) /c– (a–b · (1–η₂) ) ) /b                     (III) ,

                                          0.6 ≤ c < 1                                (IV) ,

preferably                                0.7 ≤ c < 1                               (IVa) ,

more preferably                          0.7 ≤ c ≤ 0.9                             (IVb) ,

particular preferably                   0.75 ≤ c ≤ 0.85                            (IVc) ,

where

c is the depth of discharge of the anode.
A method for producing a lithium-ion battery comprising a cathode, an electrolyte, and an anode, wherein the anode comprises the electrode material of claim 16, and said method includes the following steps:

1) prelithiating the active material of the anode or the anode to a prelithiation degree ε, and

2) assembling the anode and the cathode to obtain said lithium-ion battery, characterized in that the initial surface capacity a of the cathode, the initial surface capacity b of the anode, and the prelithiation degree ε satisfy the relation formulae

                               1 < (b · (1–ε) /a) ≤ 1.2                          (I) ,

preferably                  1.05 ≤ (b · (1–ε) /a) ≤ 1.15                       (Ia) ,

more preferably             1.08 ≤ (b · (1–ε) /a) ≤ 1.12                       (Ib) ,

              0 < ε ≤ ( (a · η₁) /0.6– (a–b · (1–η₂) ) ) /b                (II) ,

where

ε is the prelithiation degree of the anode,

η₁ is the initial coulombic efficiency of the cathode, and

η₂ is the initial coulombic efficiency of the anode.
The method of claim 21, characterized in that

                ε＝ ( (a · η₁) /c– (a–b · (1–η₂) ) ) /b                      (III) ,

                                         0.6 ≤ c < 1                                (IV) ,

preferably                               0.7 ≤ c < 1                               (IVa) ,

more preferably                         0.7 ≤ c ≤ 0.9                             (IVb) ,

particular preferably                  0.75 ≤ c ≤ 0.85                            (IVc) ,

where

c is the depth of discharge of the anode.
A lithium-ion battery comprising a cathode, an electrolyte, and an anode, characterized in that the anode comprises the electrode material of claim 16, and said lithium-ion battery is subjected to a formation process, wherein said formation process includes an initial formation cycle comprising the following steps:

a) charging the battery to a cut off voltage V_off which is greater than the nominal charge cut off voltage of the battery, preferably up to 0.8 V greater than the nominal charge cut off voltage of the battery, more preferably 0.1 ～ 0.5 V greater than the nominal charge cut off voltage of the battery, particular preferably 0.2 ～ 0.4 V greater than the nominal charge cut off voltage of the battery, especially preferably about 0.3 V greater than the nominal charge cut off voltage of the battery, and

b) discharging the battery to the nominal discharge cut off voltage of the battery.
The lithium-ion battery of claim 23, characterized in that the relative increment r of the initial surface capacity of the cathode over the nominal initial surface capacity a of the cathode and the cut off voltage V_off satisfy the following linear equation with a tolerance of±10％

r＝0.75V_off–3.134 (V) .
The lithium-ion battery of claim 23, characterized in that the relative increment r of the initial surface capacity of the cathode over the nominal initial surface capacity a of the cathode and the cut off voltage V_off satisfy the following quadratic equation with a tolerance of±10％

r＝–0.7857V_off ²+7.6643V_off–18.33 (Va) .
The lithium-ion battery of any one of claims 23 to 25, characterized in that the nominal initial surface capacity a of the cathode and the initial surface capacity b of the anode satisfy the relation formulae

                     1 < b · η₂ / (a · (1+r) –b · (1–η₂) ) –ε ≤ 1.2          (I′) ,

preferably       1.05 ≤ b · η₂ / (a · (1+r) –b · (1–η₂) ) –ε ≤ 1.15        (Ia′) ,

more preferably  1.08 ≤ b · η₂ / (a · (1+r) –b · (1–η₂) ) –ε ≤ 1.12        (Ib′) ,

                       0 < ε ≤ ( (a · η₁) /0.6– (a–b · (1–η₂) ) ) /b           (II) ,

where

ε is the prelithiation degree of the anode, and

η₂ is the initial coulombic efficiency of the anode.
The lithium-ion battery of any one of claims 23 to 26, characterized in that

                 ε＝ ( (a · η₁) /c– (a–b · (1–η₂) ) ) /b                   (III) ,

                                   0.6 ≤ c < 1                                     (IV) ,

preferably                         0.7 ≤ c < 1                                    (IVa) ,

more preferably                   0.7 ≤ c ≤ 0.9                                  (IVb) ,

particular preferably            0.75 ≤ c ≤ 0.85                                 (IVc) ,

where

η₁ is the initial coulombic efficiency of the cathode, and

c is the depth of discharge of the anode.
The lithium-ion battery of any one of claims 23 to 27, characterized in that the electrolyte comprises one or more fluorinated carbonate compounds, preferably fluorinated cyclic or acyclic carbonate compounds, as a nonaqueous organic solvent.
The lithium-ion battery of any one of claims 23 to 28, characterized in that after being subjected to the formation process, said lithium-ion battery is still charged to a cut off voltage V_off, which is greater than the nominal charge cut off voltage of the battery, preferably up to 0.8 V greater than the nominal charge cut off voltage of the battery, more preferably 0.1 ～ 0.5 V greater than the nominal charge cut off voltage of the battery, particular preferably 0.2 ～ 0.4 V greater than the nominal charge cut off voltage of the battery, especially preferably about 0.3 V greater than the nominal charge cut off voltage of the battery, and is discharged to the nominal discharge cut off voltage of the battery.
A method for producing a lithium-ion battery comprising a cathode, an electrolyte, and an anode, wherein the anode comprises the electrode material of claim 16, and said method includes the following steps:

1) assembling the anode and the cathode to obtain said lithium-ion battery, and

2) subjecting said lithium-ion battery to a formation process, wherein said formation process includes an initial formation cycle comprising the following steps:

a) charging the battery to a cut off voltage V_off which is greater than the nominal charge cut off voltage of the battery, preferably up to 0.8 V greater than the nominal charge cut off voltage of the battery, more preferably 0.1 ～ 0.5 V greater than the nominal charge cut off voltage of the battery, particular preferably 0.2 ～ 0.4 V greater than the nominal charge cut off voltage of the battery, especially preferably about 0.3 V greater than the nominal charge cut off voltage of the battery, and

b) discharging the battery to the nominal discharge cut off voltage of the battery.
The method of claim 30, characterized in that the relative increment r of the initial surface capacity of the cathode over the nominal initial surface capacity a of the cathode and the cut off voltage V_off satisfy the following linear equation with a tolerance of±10％

r＝0.75V_off–3.134 (V) .
The method of claim 30, characterized in that the relative increment r of the initial surface capacity of the cathode over the nominal initial surface capacity a of the cathode and the cut off voltage V_off satisfy the following quadratic equation with a tolerance of±10％

r＝–0.7857V_off ²+7.6643V_off–18.33 (Va) .
The method of any one of claims 30 to 32, characterized in that the nominal initial surface capacity a of the cathode and the initial surface capacity b of the anode satisfy the relation formulae

                     1 < b · η₂ / (a · (1+r) –b · (1–η₂) ) –ε ≤ 1.2          (I′) ,

preferably       1.05 ≤ b · η₂ / (a · (1+r) –b · (1–η₂) ) –ε ≤ 1.15        (Ia′) ,

more preferably  1.08 ≤ b · η₂ / (a · (1+r) –b · (1–η₂) ) –ε ≤ 1.12        (Ib′) ,

                       0 < ε ≤ ( (a · η₁) /0.6– (a–b · (1–η₂) ) ) /b           (II) ,

where

ε is the prelithiation degree of the anode, and

η₂ is the initial coulombic efficiency of the anode.
The method of any one of claims 30 to 33, characterized in that

                 ε＝ ( (a · η₁) /c– (a–b · (1–η₂) ) ) /b                   (III) ,

                                   0.6 ≤ c < 1                                     (IV) ,

preferably                         0.7 ≤ c < 1                                    (IVa) ,

more preferably                   0.7 ≤ c ≤ 0.9                                  (IVb) ,

particular preferably            0.75 ≤ c ≤ 0.85                                 (IVc) ,

where

η₁ is the initial coulombic efficiency of the cathode, and

c is the depth of discharge of the anode.
The method of any one of claims 30 to 34, characterized in that the electrolyte comprises one or more fluorinated carbonate compounds, preferably fluorinated cyclic or acyclic carbonate compounds, as a nonaqueous organic solvent.