WO2008065984A1 - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

Disclosed is a nonaqueous electrolyte secondary battery comprising a positive electrode, a negative electrode and a nonaqueous electrolyte solution obtained by dissolving a lithium salt into a nonaqueous solvent. The negative electrode has a collector and an active material layer which is formed on at least one side of the collector and contains an active material containing Si. The nonaqueous solvent contains compounds represented by the formula (1) below and the formula (2) below. The ratio of the compound represented by the formula (1) below in the nonaqueous solvent is 15-40% by volume. In the compound represented by the formula (1) below, it is preferable that only one of R1 and R2 is F, and the other is H. [chemical formula 1] (1) In the formula, R1 and R2 respectively represent H or F, and at least one of R1 and R2 represents F. (2) In the formula R3-R6 respectively represent H or F.

Description

 Specification

 Non-aqueous electrolyte secondary battery

 Technical field

 [0001] The present invention relates to a non-aqueous electrolyte secondary battery such as a lithium secondary battery.

 Background art

 [0002] Graphite is generally used as a negative electrode active material of a lithium secondary battery. However, with the recent increase in functionality of electronic devices, their power consumption has increased remarkably, and large-capacity secondary batteries have become more and more necessary. Meeting that need is difficult. Therefore, development of negative electrode active materials made of Si-based materials, which are materials with a higher capacity than graphite, has been actively conducted. However, a negative electrode active material made of a Si-based material is likely to be pulverized due to a large degree of expansion / contraction caused by charge / discharge.

 [0003] By the way, it is known that a Si-based negative electrode active material decomposes an electrolytic solution on the surface of a battery when it is charged. This decomposition forms a film called SEI (solid electrolyte inteface) on the surface of the active material. The formation of SEI is an irreversible reaction, and once SEI is formed on the surface of the active material, it remains there. As described above, since the Si-based negative electrode active material is easily pulverized with charge and discharge, a new surface, that is, a surface on which SEI is easily formed is generated each time the charge and discharge are performed. SEI is formed on this new surface, and new SEI may be deposited on the already formed SEI. As a result, the lithium ion reaches the active material surface with repeated charge / discharge and is gradually hindered by SEI, and the electrode reaction is inhibited. Also, the formation of SEI may reduce the amount of electrolyte. For these reasons, it is not easy to improve the cycle characteristics of the battery.

[0004] From the viewpoint of preventing decomposition of the electrolytic solution, it has been proposed to use 4-fluoroethylene carbonate as a non-aqueous solvent used in the electrolytic solution (see Patent Documents 1 and 2). In these patent documents, 4 fluoro-ethylene carbonate is used in combination with dimethyl carbonate and dimethyl carbonate, which are linear carbonates. In these documents, however, 4-fluoro-ethylene carbonate and linear Nate is used so that the blending ratio of both is about 1: 1, so that the viscosity of the electrolyte is increased and the rate characteristics are likely to be lowered.

 [0005] Patent Document 1: US2006 / 0099515A1

 Patent Document 2: US2006 / 0134524A1

 Disclosure of the invention

 [0006] The present invention relates to a non-aqueous electrolyte secondary battery having a non-aqueous electrolyte obtained by dissolving a lithium salt in a positive electrode, a negative electrode, and a non-aqueous solvent.

 The negative electrode has an active material layer including a current collector and an active material containing Si formed on at least one surface thereof,

 The non-aqueous solvent includes a compound represented by the following formula (1) and a compound represented by the following formula (2), and the ratio of the compound represented by the formula (1) in the non-aqueous solvent is 15 to 40 The present invention provides a non-aqueous electrolyte secondary battery characterized by being in volume%.

[0007] [Chemical 1]

式中、 R¹及び R²は H又は Fを表し、

Wherein R ¹ and R ² represent H or F,

At least one of R ¹ and R ² is F

 It is.

式中、 R³〜R⁶は H又は Fを表す。 図面の簡単な説明

In the formula, R ^{3 to} R ⁶ represent H or F. Brief Description of Drawings

 FIG. 1 is a schematic view showing a cross-sectional structure of an embodiment of a negative electrode used in a nonaqueous electrolyte secondary battery of the present invention.

FIG. 2 is a process diagram showing a method for producing the negative electrode shown in FIG. BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described based on preferred embodiments thereof. The non-aqueous electrolyte secondary battery of the present invention (hereinafter also simply referred to as a secondary battery or a battery) has a positive electrode, a negative electrode, and a separator disposed between them as its basic constituent members. The space between the positive electrode and the negative electrode is filled with a non-aqueous electrolyte via a separator. The battery of the present invention may be in the form of a cylinder, a square, a coin, or the like provided with these basic components. The force is not limited to these forms.

 [0010] The positive electrode used in the battery of the present invention is formed, for example, by forming a positive electrode active material layer on at least one surface of a current collector. The positive electrode active material layer contains an active material. As this active material, for example, a lithium transition metal composite oxide is used. Lithium transition metal complex oxides include LiCoO, LiNiO, LiMn O, LiMnO, LiCo Ni O, LiNi C

 2 2 2 4 2 0.5 0.5 2 0.7 o Mn 〇, LiNi Co Mn 〇, Li (Li Mn Co) O (where x is 0 <x <1/3

0.2 0.1 2 1/3 1/3 1/3 2 x 2x 1- 3x 2

 Is used). However, the force is not limited to these. These positive electrode active materials can be used singly or in combination of two or more.

 Of the various positive electrode active materials described above, those containing Li and Co as constituent elements, such as LiCoO, have the property of contracting when lithium is occluded and expanding when released. On the other hand, those containing Ni as a constituent element such as LiNiO and those containing Mn as a constituent element such as LiMn O have the ability to expand when lithium is occluded, or the degree of contraction during lithium occlusion, It is smaller than lithium transition metal complex oxides containing Li and Co as constituent elements.

 [0012] The positive electrode used in the present invention is a positive electrode mixture prepared by suspending the positive electrode active material in a suitable solvent together with a conductive agent such as acetylene black and a binder such as polyvinylidene fluoride. This is obtained by applying and drying at least one surface of a current collector such as an aluminum foil, followed by roll rolling and pressing.

[0013] When the positive electrode is produced by the above method, the lithium primary transition metal composite oxide has an average primary particle diameter of 5 m or more and 10 m or less, which is a balance between packing density and reaction area. To preferred. Polyvinylidene fluoride used as a binder should have a weight average molecular weight of 350,000 or more and 2,000,000 or less. From the point which can improve property.

 [0014] The negative electrode used in the battery of the present invention is formed, for example, by forming a negative electrode active material layer on at least one surface of a current collector. The negative electrode active material layer contains an active material. The active material used in the present invention is a substance containing Si. The secondary battery of the present invention is characterized in that a substance containing Si is used as a negative electrode active material and is combined with a nonaqueous solvent described later. Details thereof will be described later.

 [0015] The negative electrode active material containing Si is capable of occluding and releasing lithium ions. A negative electrode active material containing Si has a property of expanding when lithium is occluded and contracting when released. The degree of expansion and contraction is much larger than that of carbon-based materials that have been used as the negative electrode active material of conventional lithium secondary batteries. Examples of the negative electrode material containing Si include silicon alone, an alloy of silicon and metal, silicon oxide, silicon nitride, and silicon boride. These materials can be used alone or in combination. Examples of the metal include one or more elements selected from the group consisting of Cu, Ni, Co, Cr, Fe, Ti, Pt, W, Mo, and Au. Of these metals, Cu, Ni, and Co are preferred. In particular, Cu and Ni are desirable because they have excellent electron conductivity and low ability to form lithium compounds. A particularly preferable negative electrode active material containing Si is silicon alone or silicon oxide from the viewpoint of high lithium storage capacity.

 [0016] The negative electrode active material containing Si preferably contains lithium before starting the charging of the battery. As a result, lithium is accumulated as an irreversible capacity in the negative electrode active material before the start of charge / discharge. Therefore, since charging / discharging is started from the state where lithium is occluded in the negative electrode active material, charging / discharging is performed almost 100% reversibly. Moreover, since Si-based materials have low electron conductivity, during discharge, lithium is constantly occluded in the negative electrode active material, so that the electron conductivity is improved and negative polarity is reduced. Polarization is reduced. This makes it difficult for the voltage of the negative electrode to rapidly decrease at the end of discharge.

[0017] The fact that the negative electrode active material containing Si contains lithium before the start of battery charging means that overvoltage can be kept low during lithium insertion reaction (during charging) on the surface of the negative electrode active material. It is advantageous from the point of being. This is because the overvoltage can be kept low to prevent side reactions such as excessive SEI formation.

[0018] In order to allow lithium to be contained in the negative electrode active material containing Si before the start of charging, for example, before or after the negative electrode is incorporated into the battery and before the start of charging, by known means. What is necessary is just to occlude lithium with respect to the negative electrode active material containing Si. As means for occluding lithium, for example, the method described in WO2006 / 100794A1 and the method described in US5556721A relating to the earlier application of the present applicant can be used. Alternatively, the negative electrode before lithium occlusion and metallic lithium are immersed in a non-aqueous solvent containing a lithium salt through a mesh made of a conductive material, and the negative electrode and metallic lithium are electrochemically short-circuited. Can absorb lithium. In this case, the amount of occlusion can be controlled by the environmental temperature, mesh size, mesh material, non-aqueous solvent composition, and immersion time. Regardless of which method is employed, the lithium storage amount is preferably set to 10 to 40%, more preferably 20 to 40%, and even more preferably 25 to 35% with respect to the initial charge theoretical capacity of the negative electrode active material. To do. In this embodiment, when a material containing silicon is used as the negative electrode active material, theoretically, silicon occludes lithium to a state represented by the composition formula SiLi.

 4.4

 The fact that the storage amount of lithium is 100% of the theoretical capacity of initial charge of silicon means that lithium is stored in silicon until the state expressed by the composition formula SiLi!

 4.4

 [0019] The negative electrode active material layer can be, for example, a continuous thin film layer made of the negative electrode active material. In this case, a negative electrode active material layer is formed on at least one surface of the current collector by various thin film forming means such as chemical vapor deposition, physical vapor deposition, and sputtering. The thin film may be etched to form a number of voids extending in the thickness direction. For etching, a wet etching method using a sodium hydroxide aqueous solution or the like, or a dry etching method using a dry gas or a plasma can be employed. In addition to the form of the continuous thin film layer, the negative electrode active material layer may be a coating layer containing particles of the negative electrode active material, a sintered body layer containing particles of the negative electrode active material, or the like. Further, it may be a layer having a structure shown in FIG.

As the separator in the secondary battery of the present invention, a synthetic resin nonwoven fabric, polyolefin such as polyethylene or polypropylene, or a porous film of polytetrafluoroethylene is preferably used. Is it a viewpoint to suppress the heat generation of the electrode that occurs when the battery is overcharged? Preferably use a separator in which a thin film of a phucene derivative is formed on one side or both sides of a polyolefin microporous membrane. The separator preferably has a puncture strength of 0.2 N / 〃m thickness or more and 0.3.ΘΝ / ^ πι thickness or less and a tensile strength in the winding axis direction of 40 MPa or more and 150 MPa or less. Even when using Si-based material, which is a negative electrode active material that expands and contracts greatly with charge and discharge, damage to the separator can be suppressed and the ability to control the occurrence of internal short-circuiting It is.

 [0021] The nonaqueous electrolytic solution is a solution in which a lithium salt as a supporting electrolyte is dissolved in a nonaqueous solvent.

 The secondary battery of the present invention has one of the characteristics in the type of the non-aqueous solvent. Specifically, the nonaqueous solvent contains the compounds represented by the above formulas (1) and (2). As described above, in the secondary battery of the present invention, the material containing Si is used as the negative electrode active material. However, the negative electrode active material containing Si has a property of decomposing a nonaqueous solvent during charging. is doing. From the viewpoint of preventing this decomposition, in the present invention, a non-aqueous solvent containing each of the above compounds, that is, a fluorinated non-aqueous solvent is used.

[0022] When the non-aqueous solvent used in the lithium secondary battery is generally fluorinated, the viscosity thereof is higher than before fluorination due to the bulky steric hindrance of the fluorine atom. In addition, the introduction of fluorine with strong electronegativity increases the polar and intermolecular interactions. A stronger intermolecular interaction leads to an increase in viscosity. An increase in polarity leads to an increase in dielectric constant. An increase in viscosity causes an increase in the internal resistance of the battery due to a decrease in conductivity, which acts in the negative direction, such as a decrease in rate characteristics. On the other hand, an increase in the dielectric constant acts in the positive direction of increasing the conductivity. Comparing the negative effect resulting from the increase in the viscosity and the positive effect resulting from the increase in the dielectric constant, the negative effect resulting from the increase in the viscosity tends to be larger. Therefore, if the amount of the fluorinated nonaqueous solvent is reduced to prevent an increase in the viscosity, the conductivity of the whole nonaqueous mixed solvent is lowered and the internal resistance of the battery is increased. This is because the compound itself represented by the formula (1) has a characteristic that the dielectric constant is higher than that of the compound represented by the formula (2). Considering these circumstances, in the present invention, the ratio of the compound represented by the formula (1) in the non-aqueous medium is set to 15 to 40% by volume, which is lower than the conventional value, preferably 20 to 40% by volume, more preferably 25-40% by volume, more preferably Is set to 25-35% by volume. As a result, it is possible to prevent decomposition of the nonaqueous solvent and thus improve cycle characteristics while suppressing an increase in the internal resistance of the battery and a decrease in the charge / discharge rate characteristics. On the other hand, the proportion of the compound represented by the formula (2) in the non-aqueous solvent is set to 55 to 85% by volume, which is a higher rate than before, preferably 55 to 80% by volume, more preferably 55 to It is set to 75% by volume, more preferably 65 to 75% by volume. The compound represented by the formula (2) is a substance having a lower viscosity than the compound represented by the formula (1). Therefore, by increasing its use ratio, the non-aqueous electrolyte as a whole can be obtained. Increase in viscosity is suppressed. As used herein, “volume” means the volume at 25 ° C.

In the present embodiment, in addition to the compound represented by the formula (1) and the compound represented by the formula (2), other nonaqueous solvents conventionally used in nonaqueous electrolyte secondary batteries May also be used. For example, a compound in which R ¹ and R ² are both H in formula (1) may be used. The amount of the other non-aqueous solvent that can be optionally used in combination is preferably from! To 50%, particularly from 10 to 30%, based on the total volume of the non-aqueous solvent.

As a result of the study by the present inventors, in the compound represented by the formula (1), when only ^one of R ¹ and R ² is F and the other is H, that is, the formula ( It was found that when the compound represented by 1) is a monofluorinated compound, its decomposition is further prevented and the cycle characteristics are further improved. For the same reason, it was found that it is preferable that all of R ^{3 to} R ⁶ are H for the compound represented by the formula (2). In addition, when F is contained in the compound represented by the formula (1), stable SEI is formed on the surface of the negative electrode active material when the battery is charged. LiF and other elements are formed in this SEI, and as a result, the Li part responsible for charging and discharging is consumed. Therefore, it is advantageous that the negative electrode active material contains Li before the start of charge / discharge of the battery from the viewpoint that the Li becomes a supply source of consumed Li.

[0025] With respect to the compound represented by the formula (2), the compound is a compound in which R ^{3 to} R ⁶ are H in the same formula and a compound in which at least one of R ^{3 to} R ⁶ is F in the same formula It is also preferable to be a mixture thereof. This is because, by using such a mixture, the capacity retention ratio is further improved as compared with the case where only the compound in which R ³ to R ⁶ are H in the compound represented by the formula (2). When using such a mixture, from this viewpoint, in the compound represented by the formula (2), the proportion of the compound in which at least one of R ^{3 to} R ⁶ is F in the non-aqueous solvent is 10 to 40. The force S is preferably%, and more preferably 15 to 30% by volume. On the other hand, the proportion of the compound represented by formula (2) in which R ^{3 to} R ⁶ are H in the non-aqueous solvent is preferably 35 to 65% by volume, and is preferably 45 to 55% by volume. More preferably.

[0026] In particular, when the above mixture is used in combination with the compound represented by the formula (1), in which only ^one of R ¹ and R ² is F and the other is H. This is preferable because the capacity retention rate is further improved.

[0027] When the compound represented by the formula (2) includes F, it is preferable that only one of R ^{3 to} R ⁶ is F. In particular, it is preferred that R ³ and R ⁶ bonded to the carbon located at the terminal in the molecular skeleton! /, And only one of them is F! /.

 In the present embodiment, 1,3-dimethyl-2-imidazolidinone (hereinafter referred to as DMI) is used in addition to the compound represented by formula (1) and the compound represented by formula (2). I also like it. By using DMI together, the capacity maintenance rate of the battery can be improved. The amount of DMI used in the non-aqueous medium is 0.5 to 1% by volume. This is preferable from the viewpoint of improving the capacity retention rate of the battery while controlling the decomposition behavior of DMI in the battery.

 [0029] As described above, in the present invention, the non-aqueous solvent on the surface of the substance containing Si used as the negative electrode active material can be obtained by using the compounds represented by the formulas (1) and (2) as the non-aqueous solvent. The decomposition of the solvent can be effectively prevented. By the way, as described above, it is preferable that the negative electrode active material containing Si is in a state containing lithium from the start of charging of the battery. Since the overvoltage during lithium insertion into the active material can be kept low, the SEI formed on the surface of the negative electrode active material tends to be thin and dense. The formation of such a thin and dense SEI suppresses the contact between a new active material surface and a non-aqueous solvent that occurs each time charging / discharging, so that continuous decomposition of the non-aqueous solvent is difficult to occur. Therefore, when the negative electrode active material containing Si contains lithium before the start of battery charging, the compounds represented by the formulas (1) and (2) are used in combination as non-aqueous solvents. The effect becomes even more remarkable.

[0030] As described above regarding the negative electrode active material containing Si, this active material has a property of expanding when lithium is absorbed and contracting when released. On the other hand, as described above for the positive electrode active material, the lithium transition metal composite oxide containing Li and Co as constituent elements Has the property of contracting when lithium is occluded and expanding when released. In other words, the lithium transition metal composite oxide and the negative electrode active material containing Si exhibit the opposite behavior in terms of volume change with respect to the insertion and release of lithium. Therefore, when the battery of the present invention is charged, the negative electrode active material expands and the positive electrode active material contracts. Conversely, during discharge, the positive electrode active material expands and the negative electrode active material contracts. Thus, during charging, the volume of the expansion of the negative electrode active material is absorbed by the volume of contraction of the positive active material, and conversely, during the discharge, the volume of expansion of the positive active material is negative. Absorbed by the volume of contraction of the active material. As a result, in both cases of charging and discharging, it is possible to suppress the generation of stress due to the volume change of the entire battery, and effectively prevent the positive electrode, the negative electrode, the separator, etc. from being damaged. Is done. As a result, the cycle characteristics of the battery are improved. As described above, in the present invention, by using a specific positive electrode active material, in addition to improving cycle characteristics by preventing decomposition of the water-insoluble medium, it is possible to suppress generation of stress due to volume change of the entire battery. The cycle characteristics can also be improved.

 [0031] In particular, the weight ratio of Co in the positive electrode (Co / Si) to Si in the negative electrode is preferably 0.3 to 5.5, more preferably 0.4 to 2.5, and still more preferably 0.4 to ;! By setting to, the generation of stress due to the volume change of the entire battery can be suppressed more effectively. Specifically, by setting the Co / Si weight ratio to 0.3 or more, the deterioration of the rate characteristics due to the decrease in the amount of Co is avoided, and the negative electrode also has a high energy density (or capacity). Will be obtained. In addition, by setting the Co / Si weight ratio to 5.5 or less, the balance between the contraction of the positive electrode active material and the expansion of the negative electrode active material during charging is improved, and the stress is sufficiently relaxed. .

 [0032] LiCoO is a lithium transition metal composite oxide containing Li and Co as constituent elements.

 The composite oxide that can be used in the present invention is not limited to this, and, for example, a compound represented by the following formula (1) can be widely used.

 LiCo M O (1)

 1-2

 (In the formula, X represents a positive number less than 1, and M represents a metal element.)

In the above formula, examples of the metal element represented by M include transition metal elements other than Co and typical metal elements other than Li. Examples of transition metal elements include Ni, Mn, Fe, Examples include V, Zr, Ti, Mo, W, and Nb. In particular, it is preferable to use Ni and / or Mn as the transition metal element. On the other hand, examples of typical metal elements include Mg, Al, and Ga. As described above, X in the composite oxide is a positive number less than 1, preferably (0.1 to 0.4, more preferably 0.1 to 0.30).

[0034] The amount of Co contained in the composite oxide is preferably 10 to 40% by weight, more preferably 10 to 30% by weight, based on the total amount of transition metal elements contained in the composite oxide. It is. By setting the amount of Co in the composite oxide within this range, it is possible to make the degree of shrinkage when the composite oxide occludes lithium within an appropriate range.

 [0035] As a specific example of the composite oxide, for example, Li (Co Mn Ni) O (wherein a + b + c

= 1, 0 <a <l, 0≤b <l, 0≤c <l, where b and c cannot be 0 on the same day temple: Li (Co Fe

) 0 (where a + b = l, 0 <a <l, 0 <b <l), Li (Co V) 0 (where a + b = l, 0 and a

<1, 0 <b <l). These composite oxides may be used alone or in combination of two or more. Of these composite oxides, it is preferable to use materials that have a large volume change due to the storage and release of lithium. The reason is that the material containing Si, which is the negative electrode active material used in combination with the composite oxide, is a material that has a large volume change due to the insertion and release of lithium. This is because volume changes of the entire battery are easily offset. Preferred materials from this point of view include, for example, LiMn Co Ni O, LiMn Co Ni O, and LiMn Co.

Ni O etc. are mentioned. These materials are produced by a known method, for example, by baking lithium carbonate and a transition metal oxide in the atmosphere.

 [0036] From the viewpoint of further improving the cycle characteristics of the battery, it is preferable to use a negative electrode having the structure shown in FIG. The negative electrode 10 of the embodiment shown in the figure includes a current collector 11 and an active material layer 12 formed on at least one surface thereof. In FIG. 1, for the sake of convenience, the active material layer 12 is shown on only one side of the current collector 11, and the active material layer is formed on both sides of the current collector! Yo!

In the active material layer 12, at least a part of the surface of the active material particles 12 a containing Si is coated with a metal material having a low lithium compound forming ability. This metal material 13 It is a material different from the constituent material of the child 12a. Voids are formed between the particles 12a coated with the metal material. That is, the metal material covers the surfaces of the particles 12a in a state where a gap is secured so that the non-aqueous electrolyte containing lithium ions can reach the particles 12a. In FIG. 1, the metal material 13 is conveniently represented as a thick line surrounding the particle 12a. Each particle is in direct contact with other particles or through a metal material 13. “Lithium compound forming ability is low” means that lithium does not form an intermetallic compound or a solid solution, or even if lithium is formed, the amount of lithium is very small or very unstable.

 [0038] The metal material 13 has conductivity, and examples thereof include copper, nickel, iron, cobalt, and alloys of these metals. In particular, the metal material 13 is preferably a highly ductile material in which even if the active material particles 12a expand and contract, the coating on the surface of the particles 12a is difficult to break! It is preferable to use copper as such a material.

 The metal material 13 is preferably present on the surface of the active material particles 12 a over the entire thickness direction of the active material layer 12. The active material particles 12 a are preferably present in the matrix of the metal material 13. Accordingly, even if the particles 12a are pulverized due to expansion / contraction due to charge / discharge, the particles are less likely to fall off. In addition, since the electronic conductivity of the entire active material layer 12 is ensured through the metal material 13, the electrically isolated active material particles 12 a are generated, particularly in the deep part of the active material layer 12. The formation of the active material particles 12a is effectively prevented. The presence of the metal material 13 on the surface of the active material particles 12a over the entire thickness direction of the active material layer 12 can be confirmed by electron microscope mapping using the material 13 as a measurement target.

[0040] The metal material 13 covers the surfaces of the particles 12a continuously or discontinuously. When the metal material 13 continuously covers the surfaces of the particles 12a, it is preferable to form fine voids in the coating of the metal material 13 so that a nonaqueous electrolytic solution can flow. When the metal material 13 discontinuously covers the surface of the particle 12a, the non-aqueous electrolyte is supplied to the particle 12a through a portion of the surface of the particle 12a that is not covered with the metal material 13. . In order to form the coating of the metal material 13 having such a structure, the metal material 13 may be deposited on the surfaces of the particles 12a by, for example, electrolytic plating according to the conditions described later. [0041] The metal material 13 covering the surface of the active material particles 12a has an average thickness of preferably 0.05 to 2111, more preferably 0.1 to 0.25 in. It is. That is, the metal material 13 covers the surface of the active material particles 12a with a minimum thickness. This prevents the dropout due to the particles 12a from expanding and contracting due to charge and discharge to be pulverized while increasing the energy density. Here, the “average thickness” is a value calculated based on a portion of the surface of the active material particle 12 a that is actually covered with the metal material 13. Accordingly, the portion of the surface of the active material particle 12a that is not covered with the metal material 13 is not used as the basis for calculating the average value.

 [0042] The surface of the active material particle 12a is coated with the metal material 13, whereby electron conductivity is imparted to the particle 12a. As a result, it is possible to keep the overvoltage during charging low. As a result, the SEI formed on the surface of the active material particles 12a can be made thin and precise. As mentioned earlier, the formation of such a thin and dense SEI suppresses the contact between the new active material surface and the non-aqueous solvent that occurs each time charging and discharging, and therefore, the continuous decomposition of the non-aqueous solvent. Is difficult to occur. Therefore, when the surfaces of the active material particles 12a are coated with the metal material 13, the effect of the present invention can be further improved by using the compounds represented by the formulas (1) and (2) together as the non-aqueous solvent. It will be remarkable.

 [0043] The voids formed between the particles 12a coated with the metal material 13 serve as a flow path for the non-aqueous electrolyte containing lithium ions. Since the non-aqueous electrolyte smoothly flows in the thickness direction of the active material layer 12 due to the presence of the voids, it is possible to improve the cycle characteristics. Further, the voids formed between the particles 12a also serve as a space for relieving the stress caused by the volume change of the active material particles 12a due to charge and discharge. The increase in the volume of the active material particles 12a whose volume has been increased by charging is absorbed in the voids. As a result, the particles 12a are less likely to be pulverized, and significant deformation of the negative electrode 10 is effectively prevented.

 [0044] As described later, the active material layer 12 preferably has a predetermined plating bath applied to a coating film obtained by applying a slurry containing particles 12a and a binder onto a current collector and drying the slurry. It is formed by performing the electrolytic plating used and depositing the metal material 13 between the particles 12a.

[0045] In order to form necessary and sufficient voids in the active material layer 12 in which the non-aqueous electrolyte solution can flow. For this purpose, it is preferable that the plating solution is sufficiently permeated into the coating film. In addition to this, it is preferable that the conditions for depositing the metal material 13 by electrolytic plating using the plating solution are appropriate. The plating conditions include the composition of the mating bath, the pH of the plating bath, and the current density of the electrolysis. Regarding the pH of the plating bath, it is preferable to adjust this to 7.;! ~ 11. By keeping the pH within this range, the dissolution of the active material particles 12a is suppressed, the surface of the particles 12a is cleaned, and plating on the particle surface is promoted. Gaps are formed. The pH value was measured at the plating temperature.

 [0046] When copper is used as the metal material 13 for plating, it is preferable to use a copper pyrophosphate bath. When nickel is used as the metal material, for example, an alkaline nickel bath is preferably used. In particular, it is preferable to use a copper pyrophosphate bath because the voids can be easily formed over the entire thickness direction of the layer even when the active material layer 12 is thickened. In addition, the metal material 13 is deposited on the surface of the active material particles 12a, and the metal material 13 is less likely to be deposited between the particles 12a, so that the voids between the particles 12a are successfully formed. This is also preferable. When a copper pyrophosphate bath is used, the bath composition, electrolysis conditions and pH are preferably as follows.

 'Copper pyrophosphate trihydrate: 85 ~; 120g / l

 -Pyrogin power! ; Kum: 300 ~ 600g / l

 'Potassium nitrate: 15-65g / l

 • Bath temperature: 45-60 ° C

• Current density:;! ~ 7A / dm ²

 • pH: Adjust the pH to ρΗ7 · 9 · 5 by adding ammonia water and polyphosphoric acid.

[0047] Particularly when a copper pyrophosphate bath is used, it is preferable to use one having a P ratio defined by a ratio (PO / Cu) of 5% to 12% by weight of copper and copper. When the P ratio is less than 5, the metal material 13 covering the active material particles 12a tends to be thick, and it may be difficult to form desired voids between the particles 12a. In addition, if a P ratio exceeding 12 is used, the current efficiency is deteriorated and gas generation is likely to occur, which may reduce the production stability. A more preferable copper pyrophosphate bath having a P ratio of 6.5-10.5 When used, the size and power S of the voids formed between the active material particles 12 a and the flow of the non-aqueous electrolyte in the active material layer 12 are very advantageous.

[0048] When an alkaline nickel bath is used, the bath composition, electrolysis conditions, and pH are preferably as follows.

 • Nickel sulfate: 100 ~ 250g / l

 'Ammonium chloride: 15-30g / l

 • Boric acid: 15-45g / l

 • Bath temperature: 45-60 ° C

• Current density:;! ~ 7A / dm ²

• pH: 25 weight _^0/0 aqueous ammonia: 100~300g / l pH8~ in the range of; adjusted to be 11.

 When this alkaline nickel bath is compared with the copper pyrophosphate bath described above, the use of the copper pyrophosphate bath tends to form appropriate voids in the active material layer 12, thereby extending the life of the negative electrode. It ’s easy to plan, so I like it!

 [0049] The properties of the metal material 13 can be adjusted as appropriate by adding various additives used in electrolyte solutions for producing copper foil such as proteins, active sulfur compounds, and cellulose to the various baths. It is.

[0050] The ratio of voids in the entire active material layer formed by the various methods described above, that is, the void ratio, is preferably about 15 to 45% by volume, particularly about 20 to 40% by volume. By setting the porosity within this range, it is possible to form necessary and sufficient voids in the active material layer 12 through which the non-aqueous electrolyte can flow. The void amount of the active material layer 12 is measured by a mercury intrusion method (JIS R 1655). The mercury intrusion method is a method for obtaining information on the physical shape of a solid by measuring the size and volume of pores in the solid. The principle of the mercury intrusion method is to apply pressure to mercury and press it into the pores of the object to be measured, and measure the relationship between the pressure applied at that time and the volume of mercury that has been pushed in (intruded). In this case, mercury is infiltrated sequentially from the large voids existing in the active material layer 12. In the present invention, the void amount measured at a pressure of 90 MPa is regarded as the total void amount. The porosity (%) of the active material layer 12 is the amount of voids per unit area measured by the above method. Divide by the apparent volume of the active material layer 12 and multiply by 100.

[0051] In the negative electrode of this embodiment, in addition to the porosity calculated from the void amount of the active material layer 12 measured by the mercury intrusion method being within the above range, mercury intrusion at l OMPa is performed. The porosity calculated from the void amount of the active material layer 12 measured by the method is preferably 10 to 40%. Further, it is preferable that the porosity calculated from the void amount of the active material layer 12 measured by mercury porosimetry at I MPa is 0.5 to 15%. Further, it is preferable that the porosity calculated from the void amount of the active material layer 12 measured by the silver press-in method at 5 MPa is 1 to 35%. As described above, in the mercury intrusion measurement, the mercury intrusion conditions are gradually increased. Under low pressure conditions, mercury is injected into large voids, and under high pressure conditions, mercury is injected into small voids. Therefore, the porosity measured at pressure I MPa is mainly derived from large voids. On the other hand, the porosity measured at pressure l OMPa also reflects the presence of small voids.

The force S can be controlled by appropriately selecting the particle size of the active material particles 12a. In this respect, the particle 12a has a maximum particle size of preferably 30 m or less, more preferably 10 in or less. In addition, when the particle size is expressed by D value, it is 0.

 50

 1 to 8 111, particularly 0.3 to 4 111 is preferred. The particle size of the particles is measured by laser diffraction / scattering particle size distribution measurement and electron microscope observation (SEM observation).

 [0053] If the amount of the active material relative to the entire negative electrode is too small, it is difficult to sufficiently increase the energy density of the battery. On the other hand, if the amount is too large, the strength tends to decrease and the active material tends to fall off. Considering these, the thickness of the active material layer is 10 to 40 Hm, preferably 15 to 30 μm, and more preferably 18 to 25 μm.

In the negative electrode 10 of the present embodiment, a thin surface layer (not shown) may be formed on the surface of the active material layer 12. Further, the negative electrode 10 may not have such a surface layer. The thickness of the surface layer is as thin as 0 · 25 m or less, preferably 0.1 m or less. There is no limit to the lower limit of the thickness of the surface layer. By forming the surface layer, the pulverized active material particles 12a can be further prevented from falling off. However, in this embodiment, by setting the porosity of the active material layer 12 within the above-described range, it is possible to sufficiently prevent the pulverized active material particles 12a from dropping without using a surface layer. Is possible. [0055] The negative electrode 10 has the above-mentioned thickness! /, Or has a surface layer or has the surface layer! // !!, whereby a secondary battery is assembled using the negative electrode 10, and the battery The overvoltage when performing initial charging of can be reduced. This means that lithium can be prevented from being reduced on the surface of the negative electrode 10 when the secondary battery is charged. The reduction of lithium leads to the generation of dendrites that cause short circuits between the two electrodes.

 [0056] When the negative electrode 10 has a surface layer, the surface layer covers the surface of the active material layer 12 continuously or discontinuously. When the surface layer covers the surface of the active material layer 12 continuously, the surface layer has a large number of fine voids (not shown) that are open in the surface and communicate with the active material layer 12. It is preferable. It is preferable that the fine voids exist in the surface layer so as to extend in the thickness direction of the surface layer! /. The fine voids allow the non-aqueous electrolyte to flow. The role of the fine voids is to supply a nonaqueous electrolytic solution into the active material layer 12. When the surface of the negative electrode 10 is viewed in plan by an electron microscope, the fine voids are the ratio of the area covered with the metal material 13, that is, the coverage is 95% or less, particularly 80% or less, particularly 60% or less. Such a size is preferable. If the coverage exceeds 95%, it is difficult for non-aqueous electrolytes with high viscosity to penetrate, and the selection range of non-aqueous electrolytes may be narrowed.

[0057] The surface layer is made of a metal material having a low lithium compound forming ability. This metal material may be the same as or different from the metal material 13 present in the active material layer 12. The surface layer may have a structure of two or more layers made of two or more different metal materials. Considering the ease of production of the negative electrode 10, it is preferable that the metal material 13 present in the active material layer 12 and the metal material constituting the surface layer are the same type.

[0058] The negative electrode 10 of the present embodiment has a high porosity in the active material layer 12, and therefore has high resistance to bending. Specifically, the MIT folding resistance measured according to JIS C 6471 is preferably 30 times or more, and more preferably 50 times or more. The high folding resistance is extremely advantageous since the negative electrode 10 is folded when the negative electrode 10 is folded or wound and accommodated in the battery container. As an MIT folding device, for example, a film folding fatigue tester with a tank manufactured by Toyo Seiki Seisakusho (Part No. 54 9) is used. We measure with power to measure.

 [0059] The current collector 11 in the negative electrode 10 may be the same as that conventionally used as the current collector of the negative electrode for a non-aqueous electrolyte secondary battery. The current collector 11 is composed of a metal material having a low ability to form a lithium compound as described above! /, A power of S being preferred! /. Examples of such metal materials are as already mentioned. In particular, it is preferably made of copper, nickel, stainless steel or the like. Also, it is possible to use a copper alloy foil represented by Corson alloy foil. Further, as the current collector, a metal foil having a normal tensile strength (JIS C 2318) of preferably 500 MPa or more, for example, a copper film layer formed on at least one surface of the aforementioned Corson alloy foil can be used. It is also preferable to use a current collector with a normal elongation (JIS C 2318) of 4% or more. This is because, when the tensile strength is low and the stress generated when the active material expands, cracks occur, and when the elongation is low, the current collector may crack. By using these current collectors, it is possible to further improve the folding resistance of the negative electrode 10 described above. The thickness of the current collector 11 is preferably 9 to 35 111 in consideration of the balance between maintaining the strength of the negative electrode 10 and improving the energy density. In the case where a copper foil is used as the current collector 11, it is preferable to perform a chromate treatment or an antifungal treatment using an organic compound such as a triazole compound or an imidazole compound.

 Next, a preferred method for producing the negative electrode 10 of the present embodiment will be described with reference to FIG. In this production method, a coating film is formed on the current collector 11 using a slurry containing active material particles and a binder, and then the coating is electrolyzed.

 First, a current collector 11 is prepared as shown in FIG. 2 (a). Then, a slurry containing active material particles 12 a is applied onto the current collector 11 to form a coating film 15. The surface roughness of the coating film forming surface of the current collector 11 is preferably 0.5 to 4111 at the maximum height of the contour curve. If the maximum height exceeds 4 inches, the formation accuracy of the coating film 15 is lowered, and current concentration tends to occur at the protrusions. When the maximum height is less than 0.5 111, the adhesion of the active material layer 12 tends to be lowered. As the active material particles 12a, those having the above-described particle size distribution and average particle size are preferably used.

[0062] The slurry contains a binder and a diluting solvent in addition to the particles of the active material. The slurry also contains a small amount of particles of conductive carbon materials such as acetylene black and graphite. You may go out. In particular, when the active material particles 12a are made of a silicon-based material, the conductive carbon material is preferably contained in an amount of! To 3% by weight with respect to the weight of the active material particles 12a. When the content of the conductive carbon material is less than 1% by weight, the viscosity of the slurry is lowered and the sedimentation of the active material particles 12a is promoted, so that it is difficult to form a good coating film 15 and uniform voids. Become. On the other hand, if the content of the conductive carbon material exceeds 3% by weight, plating nuclei concentrate on the surface of the conductive carbon material, and a good coating is formed.

[0063] As the binder, styrene butadiene rubber (SBR), polyvinylidene fluoride (PVDF), polyethylene (PE), ethylene propylene monomer (EPDM) and the like are used. As a diluting solvent, N-methylpyrrolidone, cyclohexane or the like is used. The amount of the active material particles 12a in the slurry is preferably about 30 to 70% by weight. The amount of the binder is preferably about 0.4 to 4% by weight. Diluting solvent is added to these to form a slurry.

 [0064] The formed coating film 15 has a large number of minute spaces between the particles 12a. The current collector 11 on which the coating film 15 is formed is immersed in a plating bath containing a metal material having a low lithium compound forming ability. By dipping in the plating bath, the plating solution enters the minute space in the coating film 15 and reaches the interface between the coating film 15 and the current collector 11. Under this condition, electrolytic plating is performed to deposit metal species on the surface of the particles 12a (hereinafter, this plating is also referred to as penetration plating). The penetration is performed by using the current collector 11 as a force sword, immersing the counter electrode as the anode in the plating bath, and connecting both electrodes to the power source.

 [0065] Precipitation of the metal material by penetration adhesion is preferably caused to proceed from one side of the coating film 15 to the other side. Specifically, as shown in FIGS. 2 (b) to (d), the electrolysis is performed so that the deposition of the metal material 13 proceeds from the interface between the coating film 15 and the current collector 11 toward the surface of the coating film. Make a mess. By precipitating the metal material 13 in this way, the surface of the active material particles 12a can be successfully coated with the metal material 13, and a void is successfully formed between the particles 12a coated with the metal material 13. can do.

[0066] As described above, the penetration conditions for depositing the metal material 13 include the composition of the plating bath, the pH of the plating bath, and the current density of electrolysis. Such conditions are as described above. [0067] As shown in FIGS. 2 (b) to (d), the deposition of the metal material 13 proceeds from the interface between the coating film 15 and the current collector 11 toward the surface of the coating film. When plating is performed, in the forefront portion of the precipitation reaction, fine particles 13a composed of plating nuclei of the metal material 13 are present in layers in a substantially constant thickness. As the precipitation of the metal material 13 proceeds, the adjacent fine particles 13a are combined to form larger particles, and when the deposition proceeds further, the particles are combined to continuously cover the surface of the active material particles 12a. It becomes like this.

 [0068] The penetration staking is terminated when the metal material 13 is deposited in the entire thickness direction of the coating film 15. By adjusting the end point of plating, a surface layer (not shown) can be formed on the upper surface of the active material layer 12. In this way, the target negative electrode is obtained as shown in FIG. 2 (d).

 [0069] After the penetration, the negative electrode 10 is preferably subjected to an antifouling treatment. Examples of the anti-bacterial treatment include organic anti-bacterials using triazole compounds such as benzotriazole, carboxybenzotriazole, tolyltriazole and imidazole, and inorganic anti-bacterials using cobalt, nickel, chromate and the like.

 Example

 [0070] Hereinafter, the present invention will be described in more detail by way of examples. However, the scope of the present invention is not limited to these embodiments.

[Example 1]

 (1) Manufacture of negative electrode

 A current collector made of an electrolytic copper foil having a thickness of 18 inches was acid washed at room temperature for 30 seconds. After the treatment, it was washed with pure water for 15 seconds. A slurry containing particles of silicon was applied on both sides of the current collector to a thickness of 15 to form a coating film. The composition of the slurry was particles: styrene butene rubber (binder): acetylene black = 100: 1 · 7: 2 (weight ratio). The average particle diameter D of the particles was 2. The average particle size D was measured using a Microtrac particle size distribution measuring device (No. 9320—X100) manufactured by Nikkiso Co., Ltd.

[0072] The current collector on which the coating film was formed was immersed in a copper pyrophosphate bath having the following bath composition, and by electrolysis, copper penetrated into the coating film to form an active material layer. did. The electrolysis conditions were as follows. DSE was used for the anode. A DC power source was used as the power source. • Copper pyrophosphate trihydrate: 105g / l

 • Potassium pyrophosphate: 450g / l

 'Potassium nitrate: 30g / l

 • P ratio: 7.7

 • Bath temperature: 50 ° C

'Current density: 3A / dm ²

 • pH: Ammonia water and polyphosphoric acid were added and adjusted to ρΗ8.2.

[0073] The penetration staking was terminated when copper was deposited over the entire thickness direction of the coating film.

 In this way, a target negative electrode was obtained. SEM observation of the vertical cross section of the active material layer confirmed that the active material particles were covered with a copper film having an average thickness of 240 nm in the active material layer. Further, the overall porosity of the active material layer was 30%. Furthermore, the porosity under lOMPa was 29%, and the porosity under IMPa was 4%. The obtained negative electrode was punched into a diameter of 14 mm.

[0074] (2) Production of positive electrode

 LiCoO was used as the positive electrode active material. This positive electrode active material was suspended in N-methylpyrrolidone as a solvent together with acetylene black (AB) and polyvinylidene fluoride (PVdF) to obtain a positive electrode mixture. The weight ratio of the mixture was LiCoO: AB: PVdF = 88: 6: 6. This positive electrode mixture was applied to a current collector made of aluminum foil (thickness 20 am) using an applicator, dried at 120 ° C, and then roll-pressed with a load of 0.2 ton / cm to obtain a positive electrode. It was. The thickness of this positive electrode was about 70 μm. This positive electrode was punched into a diameter of 13 mm.

[0075] (3) Production of lithium secondary battery

The positive electrode and the negative electrode thus obtained were opposed to each other with a separator made of a polyethylene porous film having a thickness of 20 m interposed therebetween. As a non-aqueous solvent in the electrolytic solution, a compound represented by formula (1) in which R ¹ is F and R ² is H (hereinafter referred to as F—EC) and a compound represented by formula (2) O! /, In which all of R ^{3 to} R ⁶ are H (hereinafter, DEC and! /, U) and the compound represented by formula (2), only R ³ is F A mixed solvent in which a mixture (hereinafter F-DEC and! /, U) was mixed at the ratio shown in Table 1 below was used. A solution of lmol / 1 Li PF dissolved in this mixed solvent with 2% by volume of vinylene carbonate added externally is non-aqueous. Used as electrolyte. This produced a 2032 coin cell. The weight ratio of Co / Si in the obtained battery was as shown in Table 1 below.

[Examples 2 to 4 and Comparative Examples 1 and 2]

 As a non-aqueous solvent in the non-aqueous electrolyte, a mixed solvent in which F-EC, DEC, and F-DEC are mixed in the ratio shown in Table 1 below, and the amount of the positive electrode active material and the amount of the negative electrode active material A lithium secondary battery was obtained in the same manner as in Example 1 except that the amount was changed to the value shown in the table.

[Example 5]

 Nonaqueous battery in which LiCIO is dissolved at a concentration of lmol / 1 in a nonaqueous solvent of jetyl carbonate.

 Four

 A solution was prepared. In a non-aqueous electrolyte at 25 ° C in an argon atmosphere, the negative electrode obtained in Example 2-1 and a 600 Hm-thick lithium foil were immersed through a SUS mesh (lOOmesh). The negative electrode and metallic lithium were electrochemically short-circuited. As a result, lithium was occluded in the active material in the negative electrode. The amount of lithium occluded was set to 25% of the initial charge theoretical capacity of the negative electrode active material by setting the immersion time to 40 minutes. Accurate occlusion was confirmed by ICP emission analysis. The obtained negative electrode was washed with dimethyl carbonate for the purpose of removing the lithium salt, and the solvent was removed by vacuum drying. A lithium secondary battery was obtained in the same manner as in Example 1 using the negative electrode having a negative electrode active material occluded with lithium thus obtained.

[0078] [Evaluation]

 About the lithium secondary battery obtained by the Example and the comparative example, the capacity maintenance rate and rate characteristic of the 100th cycle were evaluated by the following method. The results are shown in Table 1 below.

 [0079] [Capacity maintenance ratio at 100th cycle]

 The discharge capacity at the 13th cycle and the discharge capacity at the 100th cycle were measured. The capacity maintenance ratio at the 100th cycle was calculated by dividing the discharge capacity at the 100th cycle by the discharge capacity at the 13th cycle and multiplying by 100. Charging conditions were 0.5C and 4.2V, constant current and constant voltage. The discharge conditions were 0.5C and 2.7V, and a constant current. However, the charge / discharge at the first cycle is 0.05C, the charge / discharge at the 2nd to 4th cycle (or 0.1C, the charge / discharge at the 5th to 7th cycle (or 0.5C, 8 to; 10th cycle) Charge / discharge was 1C.

[0080] [Rate characteristics] In the capacity retention rate measurement described above, the discharge capacity at the 13th cycle was divided by the discharge capacity at the 1st cycle and multiplied by 100 to obtain the rate characteristic.

[table 1]

表 1に示す結果から明らかなように、実施例 1ないし 4の電池 (本発明品）は比較例 1及び 2の電池と同程度又はそれ以上のレート特性を有し、かつ比較例 1及び 2の電 池に比べて容量維持率が高いことが判る。つまりサイクル特性が良好であることが判 る。特に、電池の充電前から負極活物質にリチウムが吸蔵されている負極を用いた実 施例 5の電池は、容量維持率及びレート特性が極めて高くなることが判る。

As is clear from the results shown in Table 1, the batteries of Examples 1 to 4 (product of the present invention) have rate characteristics comparable to or higher than those of Comparative Examples 1 and 2, and Comparative Examples 1 and 2 It can be seen that the capacity retention rate is higher than the other batteries. In other words, it can be seen that the cycle characteristics are good. In particular, the use of a negative electrode in which lithium is occluded in the negative electrode active material before the battery is charged. It can be seen that the battery of Example 5 has extremely high capacity retention rate and rate characteristics.

Industrial applicability

 As described above, according to the present invention, it is possible to prevent decomposition of the nonaqueous solvent and deterioration of the cycle characteristics without reducing the charge / discharge rate characteristics.

Claims

A non-aqueous electrolyte secondary battery having a non-aqueous electrolyte solution in which a lithium salt is dissolved in a positive electrode, a negative electrode, and a non-aqueous solvent! /, Wherein the negative electrode comprises a current collector and at least one surface thereof An active material layer containing an active material containing Si, wherein the non-aqueous solvent includes a compound represented by the following formula (1) and a compound represented by the following formula (2): A nonaqueous electrolyte secondary battery, wherein the proportion of the compound represented by formula (1) in the nonaqueous solvent is 15 to 40% by volume.  [Chemical 1]

Wherein R ¹ and R ² represent H or F, At least one of R ¹ and R ² is F  Is

In the formula, R ^{3 to} R ⁶ represent H or F. [2] The non-aqueous electrolyte secondary according to claim 1, wherein in the compound represented by the formula (1), only ^one of R ¹ and R ² is F and the other is H. battery. 3. The nonaqueous electrolyte secondary battery according to claim 1, wherein R ^{3 to} R ⁶ are H in the compound represented by the formula (2). [4] The compound represented by the formula (2) is a compound in which R ^{3 to} R ⁶ are H in the same formula and a compound in which at least one of R ^{3 to} R ⁶ is F in the formula The nonaqueous electrolyte secondary battery according to claim 1, which is a mixture. [5] In the compound represented by the formula (2), at least one of R ^{3 to} R ⁶ is F. The non-aqueous electrolyte secondary battery according to claim 4, wherein a proportion of the non-aqueous solvent is 10 to 40% by volume.  6. The non-aqueous electrolyte secondary battery according to claim 1, wherein the Si-containing active material contains lithium before and / or starts charging of the battery. [7] The negative electrode has a negative electrode active material layer, the negative electrode active material layer contains particles of an active material containing Si, and at least a part of the surface of the particles. 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the particles are coated with a material and voids are formed between the particles coated with the metal material. [8] The positive electrode has a positive electrode active material layer containing a lithium transition metal composite oxide containing Li and Co as constituent elements,  The weight ratio of Co in the positive electrode (Co / Si) to Si in the negative electrode is 0.3 to 5. The nonaqueous electrolyte secondary battery according to claim 1, which is within the range of 5.