Lithium-ion secondary battery electrode and lithium-ion secondary battery

Abstract

Lithium-ion secondary battery electrode (20, 120, 220) comprising a foil-like electrode current collector (22, 122, 222), an electrode mixture layer (24, 124, 224) arranged on a surface of the electrode current collector (22, 122, 222) and containing a particulate electrode active material (23, 123, 223), wherein a lithium salt (25, 125) with a lithium ion uptake capacity is added to the electrode mixture layer (24, 124, 224), wherein the lithium salt (25, 125) is lithium difluorophosphate, the electrode mixture layer (24, 124, 224) is divided in its thickness direction into three equal areas (A1, A2, A3) and where, from the electrode current collector side to the surface of the electrode mixture layer (24, 124, 224), these three regions are a first region (A1), a second region (A2), and a third region (A3), the amount S1 of the lithium salt component in the first region (A1) and the amount S3 of the lithium salt component in the third region (A3) satisfy the relationship in formula (1); and a maximum particle diameter of aggregates of the lithium salt (25, 125) present in the electrode mixture layer (24, 124, 224) is less than 1 µm: 0 < S 3 / S 1 ≤ 4

Description

Cross-reference to related registrations

[0001] This application claims priority over the Japanese patent application JP 2017 - 199 478 A, filed on October 13, 2017. Background of the invention 1. Field of the invention

[0002] The present invention relates to lithium-ion secondary batteries. In particular, the present invention relates to the electrode used in lithium-ion secondary batteries and to a lithium-ion secondary battery that uses this electrode. 2. Description of the state of the art

[0003] Secondary batteries, such as lithium-ion batteries and nickel-hydride batteries, have been increasingly used in recent years as portable power sources for personal computers and portable devices, and as a source of propulsion energy for vehicles. Lithium-ion batteries, in particular, which offer high energy densities at low weights, are gaining importance as high-performance power sources (for example, the power source that drives the motor connected to a vehicle's drive wheels) used in vehicles such as electric and hybrid cars.

[0004] The lithium-ion secondary battery (hereinafter also referred to simply as "battery") consists of an electrolyte solution sandwiched between a pair of electrodes, i.e., a positive electrode and a negative electrode. An active material (positive electrode active material and negative electrode active material) is present in the ionic layer of the positive electrode and the ionic layer of the negative electrode, capable of absorbing and releasing lithium (Li). Charging / discharging such a lithium-ion secondary battery is achieved by the migration of Li ions between the positive and negative electrodes through the electrolyte solution.

[0005] For lithium-ion secondary batteries, a technique has been proposed in which a lithium salt with a high Li-ion affinity is added to the battery interior (for example, to the electrolyte solution or the mixing layer of the positive electrode). For instance, the published Japanese patent application JP 2016-51610A discloses a technique in which a first lithium salt (lithium phosphate) and a second lithium salt (e.g., lithium carbonate) are added to the mixing layer of the positive electrode, with the proportions of these lithium salts being adjusted to specific values. The technique described in published Japanese patent application JP 2016-51610A can improve the durability and internal resistance of a lithium-ion secondary battery.

[0006] WO 2014 / 184 925 A1 describes a process for producing a pasty mixture for manufacturing an electrode for a lithium-ion secondary battery. For a positive electrode, the mixture contains a lithium salt as an additive and can be applied to a metal foil as a current collector.

[0007] CN 1 03 222 104 A describes a solid-state lithium battery and its manufacture, whereby various lithium salts such as lithium halide, lithium hydride, lithium hydroxide, lithium phosphate, lithium borate, lithium nitrate, lithium sulfate, lithium vanadate and lithium oxides can be used for the electrodes. Summary of the invention

[0008] In recent years, there has been a growing demand for improvements in the performance of lithium-ion secondary batteries to enable their even more prominent use as, for example, a high-performance energy source for vehicles. There is also a desire for further improvements to the aforementioned state of the art regarding lithium salt addition.

[0009] The present invention was developed in response to this need. It is an object of the present invention to provide a technique that can advantageously improve the performance of lithium-ion secondary batteries in which a lithium salt is added to the battery interior.

[0010] To solve the aforementioned problem, as one aspect of the present invention, a lithium-ion secondary battery electrode (hereinafter also referred to simply as "electrode") with the following structure is provided.

[0011] The lithium-ion secondary battery electrode disclosed herein comprises a foil-like electrode current collector and an electrode mixing layer arranged on a surface of the electrode current collector. The electrode mixing layer contains a particulate (particle-shaped) electrode active material.

[0012] A lithium salt with lithium ion uptake capacity, namely lithium difluorophosphate, is added to the electrode mixture layer in this electrode. If the electrode mixture layer in this electrode is divided into three equal regions in the thickness direction, and these three regions are designated as a first region, a second region, and a third region, starting from the electrode current collector side and extending to the surface of the electrode mixture layer, then the amount S1 of the lithium salt component in the first region and the amount S3 of the lithium salt component in the third region satisfy the relationship shown in formula (1) below. Furthermore, the maximum particle diameter of aggregates of the lithium salt present in the electrode mixture layer of this electrode is less than 1 µm. 0 <S3 / S1≤4

[0013] In order to improve the performance of lithium-ion secondary batteries compared to those previously available, the inventor of the present invention has considered an improvement of the technology with regard to the addition of lithium salts.

[0014] As a result of various investigations, it was found that when a lithium salt was added to the battery's interior, the battery's input-output characteristics underwent significant changes as a function of the state of the lithium salt distribution in the electrode mixture layer. In particular, when, as in Fig.As shown in Figure 4, when a lithium salt is added to the electrolyte solution, the electrolyte solution readily comes into contact with the area (the third area A3) near the surface of the electrode mixture layer 124 of the electrode 120, and consequently, the lithium salt 125 readily appears in large quantities in this third area A3. When the lithium salt 125 is present in such a distorted form near the surface of the electrode mixture layer 124, the Li-ion uptake capacity is increased only in the area near the surface, and the efficient supply of Li-ions to the electrode active material 123 is impaired in the area (the first area A1) near the electrode current collector 122. As a result, the resistance in the first area A1 of the electrode mixture layer 124 experiences a local increase, and consequently, despite the addition of a lithium salt, no suitable improvement in the input-output characteristics of the battery can be achieved.

[0015] Based on this knowledge, the inventor of the present invention concluded that the local increase in the resistance of the electrode mixture layer could be suppressed if a suitable arrangement of the lithium salt in the electrode mixture layer of the first region near the electrode current collector could be achieved. As a result of various experiments, the inventor of the present invention achieved the formation of an electrode mixture layer in which the amount S1 of the lithium salt component in the first region and the amount S3 of the lithium salt component in the third region satisfied the relationship 0 < S3 / S1 ≤ 4.

[0016] In this case, a sufficient amount of the lithium salt is present in the first area near the electrode current collector, and as a result, Li ions can be efficiently supplied to this first area, and the local increase in the resistance of the electrode mixture layer can be suppressed.

[0017] Notwithstanding this improvement, the inventor of the present invention continued investigations into further improvements in battery performance. During these investigations, the inventor of the present invention concluded that the input-output characteristics of a battery are also influenced by the state of aggregation of the lithium salt and not only, as stated above, by the state of the lithium salt distribution in the thickness direction of the electrode mixture layer. In particular, and as shown in Fig.As shown in Figure 5, due to the difficulty of achieving an advantageous distribution or dispersion of the lithium salt in the electrode mixing layer, aggregates 225 of the lithium salt can form in the electrode mixing layer 224 of the electrode 220. When these aggregates 225 form, the lithium ions are attracted and concentrated towards the aggregates 225, and in such a case, the efficient supply of Li ions to the electrode active material 223 of the electrode mixing layer 224 is impaired overall, even if the presence of sufficient lithium salt near the electrode current collector 222 (the first area A1) has been achieved. As a result, the resistance of the electrode mixing layer 224 experiences a local increase, and the input-output characteristics can deteriorate.

[0018] As a result of various experiments based on this knowledge, the inventor of the present invention has discovered that the maximum particle diameter of the lithium salt aggregates must be reduced to less than 1 µm (i.e., large aggregates of equal to or greater than 1 µm are not present) in order to produce a high-performance lithium ion secondary battery electrode.

[0019] The electrode disclosed here was realized based on the knowledge presented above. The electrode mixture layer formed therein fulfills formula (1), whereby a sufficient amount of lithium salt is present in the first region near the electrode current collector and this electrode mixture layer also has a maximum particle diameter for the lithium salt aggregates of less than 1 µm.

[0020] Since lithium ions can be efficiently supplied to the entire electrode mixture layer in this lithium-ion secondary battery electrode, local increases in the resistance of the electrode mixture layer can be reliably suppressed. Accordingly, the use of this electrode in a lithium-ion secondary battery makes it possible to obtain a battery with significantly improved input-output characteristics compared to those previously available.

[0021] In this description, the "amount of lithium salt component" can be measured by separating the electrode mixture layer to provide three equal parts or portions in the thickness direction and performing ion chromatography (IC) on each of the electrode active material layers provided by this equivalent tripartite division. Additionally, the "particle diameter of the lithium salt aggregate" can be measured by taking a photograph of the cross-section of the electrode mixture layer along its thickness direction using a scanning electron microscope (SEM) and performing elemental mapping of the lithium salt in the cross-section of the electrode mixture layer by analyzing the SEM image using energy-dispersive X-ray spectroscopy (EDS).

[0022] In a preferred aspect of the lithium-ion secondary battery electrode disclosed herein, the surface of the electrode active material is coated with lithium salt in at least the first region.

[0023] By coating the surface of the electrode active material with lithium salt in this manner, lithium ions can be supplied to the electrode active material more efficiently. This, in combination with the coating, prevents reductions in battery capacity by inhibiting the decomposition of the electrolyte solution on the surface of the electrode active material. The battery performance of the lithium-ion secondary battery can thus be further improved. Particularly with regard to the first region near the electrode current collector, the effect of the lithium salt coating, as described above, can be demonstrated to be especially advantageous due to the difficulty of efficiently supplying lithium ions.

[0024] The lithium salt lithium difluorophosphate is present in the lithium-ion secondary battery electrode revealed here.

[0025] This lithium salt has a favorable absorption capacity for lithium ions and can consequently cause a beneficial reduction in the resistance of the electrode mixture layer.

[0026] Furthermore, lithium difluorophosphate can provide an additional benefit in improving battery performance. Although the reason for this is unclear, it is assumed that lithium difluorophosphate has a higher capacity for lithium ions than other lithium salts and that it also has the ability to inhibit the decomposition of the electrolyte solution on the surface of the electrode active material.

[0027] In another preferred aspect of the lithium-ion secondary battery electrode disclosed herein, the lithium salt content is 0.1 wt.% to 5 wt.%, wherein the total mass of the solid content of the electrode mixture layer is 100 wt.%.

[0028] As described above, lithium ions can be efficiently supplied to the electrode active material by ensuring that a sufficient amount of lithium salt is present in the region near the electrode current collector (the first region), without causing the formation of large aggregates. However, if the lithium salt content in the electrode mixture layer is too high, the lithium ion uptake capacity becomes excessive due to the lithium salt, and the resistance in the electrode mixture layer may instead be increased. Considering this, the lithium salt content is preferably between 0.1 wt% and 5 wt%. The lithium salt content in the electrode mixture layer can be measured by performing a quantitative analysis of the lithium salt using ion chromatography (IC).

[0029] According to another aspect of the present invention, a lithium-ion secondary battery is provided with the following structure.

[0030] The lithium-ion secondary battery disclosed herein is provided with a positive electrode in which a mixture layer of the positive electrode is formed on the surface of a current collector of the positive electrode, a negative electrode in which a mixture layer of the negative electrode is formed on the surface of a current collector of the negative electrode, and an electrolyte solution which is filled between the positive electrode and the negative electrode.

[0031] The lithium-ion secondary battery electrode according to one of the individual aspects described above is used in the battery disclosed herein for at least one of the positive electrode and the negative electrode.

[0032] With the electrode designed according to each of the aspects described above, a favorable Li-ion uptake capacity by the lithium salt can be demonstrated, since a sufficient amount of lithium salt is present in the first region near the electrode current collector and since no large aggregates with a particle diameter of 1 µm or larger are formed. As a result, Li-ions can be efficiently supplied to the entire electrode mixture layer, and a beneficial reduction in resistance in the electrode mixture layer can be achieved.

[0033] Accordingly, a lithium-ion secondary battery with significantly improved input-output characteristics can be manufactured by using an electrode that meets each of the aspects described above. Furthermore, such a lithium-ion secondary battery can be advantageously used, for example, in high-performance energy sources for vehicles.

[0034] According to another preferred aspect of the lithium-ion secondary battery disclosed herein, the lithium-ion secondary battery electrode is used for the positive electrode according to one of the individual aspects described above.

[0035] The electrode, according to each of the aspects described above, can advantageously be used for either the positive or the negative electrode in a lithium-ion secondary battery. However, when considering the inhibition of the reductive decomposition of the electrolyte solution, the electrode, according to each of the aspects described above, is preferably used for the positive electrode of a lithium-ion secondary battery. Brief description of the drawings Fig. Figure 1 is a cross-sectional diagram schematically showing the negative electrode of a lithium-ion secondary battery according to an embodiment of the present invention; Fig.Figure 2 is a perspective view that schematically shows the external shape of a lithium-ion secondary battery according to another embodiment of the present invention; Fig. Figure 3 is a perspective view that schematically shows an electrode arrangement for a lithium-ion secondary battery according to another embodiment of the present invention; Fig. Figure 4 is a cross-sectional diagram that schematically shows an example of a conventional lithium-ion secondary battery electrode; and Fig. Figure 5 is a cross-sectional diagram that schematically shows another example of a conventional lithium-ion secondary battery electrode. Detailed description of preferred embodiments

[0036] The following describes embodiments of the present invention with reference to the figures.

[0037] In this description, "lithium-ion secondary battery" refers to a secondary battery that uses Li-ion as its electrolyte and charges / discharges through the movement of the charge associated with the Li-ion between the positive and negative electrodes. Secondary batteries commonly referred to as lithium secondary batteries (or lithium-ion batteries), etc., are typical examples encompassed by the lithium-ion secondary battery in this description. 1. Lithium-ion secondary battery electrode

[0038] The following describes a lithium-ion secondary battery electrode according to an embodiment of the present invention. While the electrode disclosed herein can be used either as the positive or the negative electrode of a lithium-ion secondary battery, the description in the following embodiment mainly relates to its use as the negative electrode in a lithium-ion secondary battery.

[0039] Fig. Figure 1 is a cross-sectional diagram schematically showing the negative electrode according to the present embodiment for a lithium-ion secondary battery. Fig. Figure 1 shows the cross-section along the thickness direction Y of a mixture layer 24 of the negative electrode.

[0040] As in Fig.As shown in Figure 1, the negative electrode 20 of the present embodiment for a lithium-ion secondary battery is provided with a foil-like current collector 22 for the negative electrode and a mixing layer 24 of the negative electrode arranged on the surface of the current collector 22 for the negative electrode. This mixing layer 24 of the negative electrode contains an active material 23 of the negative electrode, which is a material capable of absorbing (or storing) and releasing the Li-ion. (1) Current collector of the negative electrode

[0041] The negative electrode current collector 22 is a foil-like, electrically conductive element. The structures and materials used for the negative electrode current collector of conventional lithium-ion secondary batteries can be used for this negative electrode current collector 22. Consequently, a detailed description has been omitted in this description; however, a copper foil with a thickness of 5 µm to 30 µm can be used as a specific example of the negative electrode current collector 22. (2) Mixing layer of the negative electrode

[0042] The mixing layer 24 of the negative electrode contains the active material 23 of the negative electrode and, in addition, additives. Besides the active material 23 of the negative electrode and additives, a lithium salt 25 is present in the mixing layer 24 of the negative electrode 20 according to the present embodiment. Each of the substances present in the mixing layer of the negative electrode is described below. (a) Active material of the negative electrode

[0043] The active material 23 of the negative electrode is a particle-like material capable of absorbing and releasing lithium ions. There are no specific restrictions regarding the material of the active material 23 of the negative electrode, and a single one of the various materials suitable for use as the active material of the negative electrode of a lithium-ion secondary battery can be used alone or in combination (mixture or composite), or two or more of these materials can be used. The following are advantageous examples of this active material 23 of the negative electrode: graphite, graphitization-resistant carbon (hard carbon), readily graphitable carbon (soft carbon), carbon nanotubes, and carbon materials with a structure that is a combination of the foregoing.With regard to energy density, the aforementioned graphite materials (natural graphite, artificial graphite, and so on) are preferably used.

[0044] Using 100 wt.% for the total mass of the solid content of the mixture layer 24 of the negative electrode, the content of the active material 23 of the negative electrode is preferably adjusted in the range of 80 wt.% to 99.45 wt.% (preferably 90 wt.% to 98.9 wt.%, for example 97 wt.%). (b) Additive

[0045] The additives that may be present in the mixing layer 24 of the negative electrode can be, for example, binders, thickeners, etc. Various polymeric materials can be used for the binder, such as styrene-butadiene rubber (SBR), polytetrafluoroethylene (PTFE), polyethylene (PE), polyacrylic acid (PAA), polyvinylidene fluoride (PVdF), and polyvinylidene chloride.

[0046] (PVdC). For the thickening agent, for example carboxymethylcellulose (CMC), methylcellulose (MC), etc. can be advantageously used.

[0047] Using 100 wt.% for the total mass of the solid content of the mixture layer 24 of the negative electrode, the additive content is preferably adjusted in the range of 0.5 wt.% to 10 wt.% (preferably 1 wt.% to 5 wt.%, for example 2 wt.%). (c) Lithium salt

[0048] Lithium salt 25 is added to the mixing layer 24 of the negative electrode 20 according to the present embodiment. This lithium salt 25 has a Li-ion uptake capacity. Since a high local charge density is assumed, particularly in areas where the lithium salt 25 is present, the Li-ion uptake capacity is increased. Therefore, when the lithium salt 25 is present in the mixing layer 24 of the negative electrode, Li-ions can be efficiently supplied to the active material 23 of the negative electrode, and a reduction in the resistance in the mixing layer 24 of the negative electrode can be achieved.

[0049] Lithium difluorophosphate is used as the lithium salt 25. Since it has a favorable capacity for accepting Li-ions, it can bring about a more advantageous reduction in the resistance of the mixing layer 24 of the negative electrode. Furthermore, the use of lithium difluorophosphate can result in a more advantageous increase in battery performance.

[0050] An advantageous lithium ion uptake capacity cannot be achieved if the content of the lithium salt 25 in the mixture layer 24 of the negative electrode is too low. Conversely, if this content is too high, the content of the active material 23 of the negative electrode will be relatively low, and the input / output properties may decrease. Taking this into account, the content of the lithium salt 25 is preferably 0.05 wt.% to 10 wt.% (more preferably 0.1 wt.% to 5 wt.%, for example 1 wt.%), when using 100 wt.% for the total mass of the solid fraction in the mixture layer 24 of the negative electrode.

[0051] Provided that the mixing layer 24 of the negative electrode is divided into three equal areas in the thickness direction Y, as in Fig.1 shown, and these three areas, starting from the side of the current collector 22 of the negative electrode to the surface of the electrode mixing layer 24 (from bottom to top in Fig. 1) where a first region A1, a second region A2 and a third region A3 are designated, the mixing layer 24 of the negative electrode in the negative electrode 20 according to the present embodiment is designed such that the amount S1 of the lithium salt component in the first region A1 and the amount S3 of the lithium salt component in the third region A3 satisfy the relationship of formula (1). 0 <S3 / S1≤4

[0052] In a negative electrode 20, for which, as stated above, the ratio (S3 / S1) of the amount S3 of the lithium salt component in the third region A3 relative to the amount S1 of the lithium salt component in the first region A1 exceeds 0 and is not greater than 4, the lithium salt 25 is present in a sufficient quantity in the region (the first region A1) near the current collector 22 of the negative electrode. Consequently, the Li ions can be efficiently supplied from the first region A1 to the active material 23 of the negative electrode. As a result, the resistance of the mixture layer 24 of the negative electrode can be advantageously reduced.

[0053] The lower limit for S3 / S1 is preferably 0.1 or more, and more preferably 0.5 or more. The upper limit is preferably 3 or less, and more preferably 2 or less. If S3 / S1 is equal to 1 (if the amount of the lithium salt component in the first region A1 is the same as in the third region A3), the lithium salt is then uniformly present over the entire mixing layer 24 of the negative electrode, and consequently, a particularly efficient supply of Li ions can be achieved.

[0054] Furthermore, the maximum particle diameter of lithium salt aggregates in the present embodiment is less than 1 µm. This prevents Li ions from being attracted and concentrated into large lithium salt aggregates of 1 µm or more. As a result, Li ions can be efficiently supplied to the entire mixture layer 24 of the negative electrode, and an even more advantageous reduction in the resistance of the mixture layer 24 of the negative electrode can be achieved.

[0055] In this respect, the lithium salt 25 is preferably not present as aggregates as mentioned above and coats the surface of the active material 23 of the negative electrode in the form of a thin film, as shown in Fig.Figure 1 shows that if the surface of the active material 23 of the negative electrode is coated in this way with a thin film of the lithium salt 25, Li-ions can be supplied even more effectively to every part of the active material 23 of the negative electrode, and in combination with this, the decomposition of the electrolyte solution on the surface of the active material can be suppressed. This can consequently contribute to a further improvement in battery performance.

[0056] As stated above, the lithium salt does not necessarily have to be the one in Fig.The thin film shown in Figure 1 is present, and it is sufficient that the lithium salt in the mixing layer of the negative electrode is in a form in which the maximum particle diameter is less than 1 µm. For example, the Li-ion can be efficiently supplied to the entire mixing layer of the negative electrode, and a beneficial reduction in the resistance of the mixing layer of the negative electrode can be achieved, even if a particulate lithium salt with a particle diameter of less than 1 µm is dispersed in the mixing layer of the negative electrode.

[0057] As stated above, the negative electrode 20 according to the present embodiment satisfies the relationship 0 <S3 / S1 ≤ 4 zwischen der Menge S1 der Lithiumsalzkomponente im ersten Bereich A1 und der Menge S3 der Lithiumsalzkomponente im dritten Bereich A3 und weist einen maximalen Teilchendurchmesser für die Lithiumsalzaggregate von weniger als 1 µm auf.

[0058] As a result, the presence of the lithium salt 25 in the mixture layer 24 of the negative electrode can be effected in a preferred distribution, whereby as a consequence Li-ions can be efficiently supplied to the active material 23 of the negative electrode of the entire mixture layer 24 of the negative electrode and the resistance of the mixture layer 24 of the negative electrode can be advantageously reduced.

[0059] Accordingly, a lithium-ion secondary battery can be obtained whose input / output characteristics are improved compared to those previously available by using the negative electrode 20 according to the present embodiment. 2. Production of a lithium-ion secondary battery electrode

[0060] The following describes methods for producing the negative electrode 20 according to the present embodiment.

[0061] Various methods can be used to manufacture the negative electrode 20 according to the present embodiment, and the present invention is therefore not limited to the manufacturing method. However, for example, a method as described below can be used.

[0062] In the production of the negative electrode for conventional lithium-ion secondary batteries, the electrode active material and additives (binders, etc.) are first dispersed in a specified solvent to create a negative electrode mixture paste. After this negative electrode mixture is applied to the surface of the negative electrode's current collector, it is dried by heat treatment. This results in a negative electrode with a mixture layer formed on the surface of the negative electrode's current collector.

[0063] In the production of the negative electrode 20 according to the embodiment described above, the lithium salt is preferably added during the production of the negative electrode mixture paste, and a salt-dissolving solvent is used for the dispersion. This makes it possible to easily produce a negative electrode 20 that fulfills formula (1) and for which the maximum particle diameter of the lithium salt aggregates is less than 1 µm.

[0064] More precisely, when a lithium salt is added to the electrolyte solution, as in Fig.As shown in Figure 4, the lithium salt 125 can separate near the surface of the electrode mixture layer 124 (the third region A3). Therefore, adding the lithium salt to the paste before the formation of the negative electrode mixture layer can be considered a means of ensuring the presence of the lithium salt near the electrode current collector 122 (the first region A1).

[0065] However, if this is done, the lithium salt can aggregate with itself to form large aggregates 225 (see Fig.5) The manufacturing process described above takes these points into account and uses a salt-dissolving solvent for the dispersion. This “salt-dissolving solvent” refers to a highly polar solvent with a dielectric constant (ε) of 40 or greater, examples of which are water (dielectric constant: 80) and propylene carbonate (dielectric constant: 65). The use of this salt-dissolving solvent allows the lithium salt to dissolve when added to the negative electrode mixture. As a result, the aggregation of the lithium salt with itself can be stopped, and the lithium salt can be advantageously dispersed in the negative electrode mixture. This leads to the formation of a mixture layer 24 of the negative electrode, which satisfies formula (1) and for which the maximum particle diameter of the lithium salt aggregates is less than 1 µm.In contrast, the lithium salt is sparingly soluble in slightly polar solvents (solvents in which salts are insoluble) such as N-methylpyrrolidone (dielectric constant: 32.2), and as a result, when using such a solvent, the lithium salt can aggregate with itself to form large aggregates 225, as in . Fig. 5 shown.

[0066] The previously described method for producing the negative electrode does not limit the present invention, and various methods can be used.

[0067] For example, in another method for producing the negative electrode 20 according to the present embodiment, the lithium salt can be bonded to each of the active material particles of the negative electrode before the negative electrode mixture paste is produced. In this case, the means for bonding the lithium salt to the active material of the negative electrode can be, for example, a vapor deposition process, such as physical vapor deposition and chemical vapor deposition, and physical bonding using, for example, a ball mill.

[0068] When such methods are used, even when using a solvent such as N-methylpyrrolidone (NMP) which does not dissolve salts, a negative electrode 20 according to the present embodiment can still be obtained for which formula (1) is satisfied and the maximum particle diameter of the lithium salt aggregates is less than 1 µm. 3. Lithium-ion secondary battery

[0069] A lithium-ion secondary battery according to another embodiment of the present invention is described below. In the lithium-ion secondary battery described in the present embodiment, the negative electrode 20 is used according to the embodiment described above.

[0070] Fig. Figure 2 is a perspective view that schematically shows the lithium-ion secondary battery according to the present embodiment. Fig. Figure 3 is a perspective view that schematically shows the electrode arrangement in the lithium-ion secondary battery according to the present embodiment.

[0071] The lithium-ion secondary battery 100 according to the present embodiment is equipped with a [missing information] as in Fig. 2 rectangular housings 50 shown are provided and are formed by arranging the in Fig.The electrode arrangement 80 shown and an electrolyte solution (not shown) inside this housing 50. Each component is described below. (1) Housing

[0072] The housing 50 consists of a flat main body 52, which is open at the top, and a cover 54, which closes this top. A positive electrode terminal 70 and a negative electrode terminal 72 are arranged in the cover 54. Although not shown in the figure, the positive electrode terminal 70 is electrically connected to the positive electrode of the electrode assembly in the housing 50, and the negative electrode terminal 72 is electrically connected to the negative electrode. (2) Electrode arrangement

[0073] In the present embodiment, the electrode arrangement 80 is as described in Fig.Figure 3 shows a wound electrode arrangement provided by winding a stack, which in turn is provided by stacking the positive electrode 10 and the negative electrode 20 with a long, sheet-shaped separator 40 arranged between them. The structure of the electrode arrangement is not shown in Figure 3. Fig. The 3 shown wound electrode arrangement is limited, and various structures can be used. An example of another structure for this electrode arrangement is a stacked electrode arrangement, as provided by stacking a variety of sheet-shaped positive and negative electrodes with a separator placed between them. (a) Positive electrode

[0074] The positive electrode 10 is provided with a long, foil-like current collector 12 and a mixing layer 14 formed on the surface (both sides) of the current collector 12. The mixing layer 14 contains an active material that participates in the uptake and release of Li ions.

[0075] A region 16 free of the mixing layer of the positive electrode, in which the mixing layer 14 of the positive electrode is not formed, is arranged with respect to the width direction at the edge on one side of the positive electrode 10. This region 16 free of the mixing layer of the positive electrode is electrically connected to the aforementioned positive electrode terminal 70 (see Fig. 3).

[0076] For the active material of the positive electrode in the lithium-ion secondary battery 100 according to the present embodiment, a lithium complex oxide is used. This lithium complex oxide can, for example, be a lithium transition metal complex oxide containing the element lithium and one, two, or more transition metal elements. Specific examples of the lithium transition metal complex oxide are lithium nickel complex oxides (e.g., LiNiO2), lithium cobalt complex oxides (e.g., LiCoO2), lithium manganese complex oxides (e.g., LiMn2O4), and lithium nickel manganese complex oxides (e.g., LiNi). 0,5 Mn 1,5 O4) and lithium nickel cobalt manganese complex oxides (for example, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3O2). For the lithium transition metal complex oxide, a polyanion-type compound (for example, an olivine type), represented by the general formula LiMPO4 (M in the formula is at least one or more transition metal elements from Co, Ni, Mn and Fe) (for example, LiFePO4, LiMnPO4), can also be used.

[0077] The “lithium-nickel-cobalt-manganese complex oxides” referred to in this description do not only refer to oxides in which the metal elements are solely Li, Ni, Co, and Mn, but also include, for example, oxides containing at least one metal element other than Li, Ni, Co, and Mn. These metal elements other than Li, Ni, Co, and Mn can be, for example, transition metals and / or main group metals (such as Al, Ca, Cr, Fe, V, Mg, Ti, Zr, Nb, Mo, W, Cu, Zn, Ga, In, Sn, La, and Ce). Similarly, the terms “lithium-nickel complex oxides,” “lithium-cobalt complex oxides,” “lithium-manganese complex oxides,” and “polyanion-type compounds” also include, for example, oxides containing at least one other metal element.

[0078] In addition to the active material of the positive electrode described above, various additives can be present in the mixture layer 14 of the positive electrode. Electrically conductive materials are one example of such additives. Preferably, carbon-containing materials, e.g., carbon powders (acetylene carbon black, etc.), carbon fibers, etc., are used for the electrically conductive material, although electrically conductive metals, e.g., nickel powder, can also be used.

[0079] Binders are another example of additives. The binder for the mixing layer 14 of the positive electrode can be the same type of polymer material as is used for the binder for the previously described mixing layer of the negative electrode. (b) Negative electrode

[0080] The negative electrode 20 is provided with a long, foil-like current collector 22 and a mixing layer 24 formed on the surface (both sides) of the current collector 22. A region 26 free of the mixing layer is arranged at the edge on one side of the negative electrode 20, relative to its width. This region 26, free of the mixing layer, is electrically connected to the previously mentioned negative electrode terminal 72 (see Figure 1). Fig. 3).

[0081] The in Fig.The negative electrode 20 shown in Figure 1 is used for the negative electrode of the lithium-ion secondary battery 100 according to the present embodiment. A detailed description is not given here, as it has already been given in the previous embodiment; however, the lithium salt 25 is introduced into the mixing layer 24 of the negative electrode 20. Additionally, a mixing layer 24 of the negative electrode is formed in which the maximum particle diameter of the lithium salt aggregates is less than 1 µm and the relationship 0 < S3 / S1 ≤ 4 of the amount S1 of the lithium salt component in the first region A1 and the amount S3 of the lithium salt component in the third region A3 is satisfied. (c) Separator

[0082] A separator 40 is positioned between the previously mentioned positive electrode 10 and the negative electrode 20. This separator 40 is made of a ribbon-like film material with a specified width and numerous micro-pores. For example, a sheet material with a single-layer or laminate structure made of a porous polyolefin resin can be used. A layer of insulating particles (a filler layer) can also be formed on the surface of this sheet material. These insulating particles can be, for example, insulating inorganic fillers (such as metal oxides and metal hydroxides) and insulating resin particles (such as particles made of polyethylene or polypropylene). (3) Electrolyte solution

[0083] The electrolyte solution (non-aqueous electrolyte solution) is housed in the casing 50 together with the aforementioned electrode assembly 80 and is located between the positive electrode 10 and the negative electrode 20, which together form the electrode assembly 80. The same non-aqueous electrolyte solutions used for conventional lithium-ion secondary batteries can be used for this electrolyte solution without any particular restriction. These non-aqueous electrolyte solutions are typically formed by introducing a conducting salt into a non-aqueous solvent.

[0084] For example, one or two or more non-aqueous solvents can be used, selected from the group consisting of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 1,3-dioxolane, etc.

[0085] For example, a lithium salt such as LiPF6, LiBF4, LiClF4, LiAsF6, LiCF3SO3, LiC4F9SO3, LiN(CF3SO2)2, LiC(CF3SO2)3 and so on can be used as a conducting salt.

[0086] The lithium-ion secondary battery 100 according to the present embodiment consists of the materials described above. Due to the use of a negative electrode 20 in which the lithium salt 25 is present in an advantageous distribution in the mixing layer 24 of the negative electrode, Li-ions can be efficiently supplied to the active material 23 of the negative electrode in the mixing layer 24 of this lithium-ion secondary battery 100. As a result, the resistance in the mixing layer 24 of the negative electrode is reduced, and the input-output characteristics are significantly improved, thus enabling particularly advantageous use in, for example, high-performance energy sources for vehicles. 4. Other embodiments

[0087] One embodiment of the present invention has been described above, but the present invention is not limited to this embodiment and various modifications can be made.

[0088] For example, the target for the preceding embodiment is the negative electrode of a lithium-ion secondary battery, but the electrode disclosed herein can also be used for the positive electrode of a lithium-ion secondary battery.

[0089] When the electrode disclosed here is used for the positive electrode, the effect of improved input-output properties can be obtained, just as when used for the negative electrode, but an effect can also be obtained in which the battery capacity is maintained due to an inhibition of the decomposition of the electrolyte solution.

[0090] If, on the other hand, the target is the negative electrode of a lithium-ion secondary battery, as in the embodiment described above, a pronounced effect can be achieved, whereby the durability properties are improved due to the formation of a coating film resulting from the reductive decomposition of the lithium salt. Accordingly, the selection of the positive or negative electrode of the lithium-ion secondary battery for the application of the electrode disclosed herein is preferably made in accordance with the respective objective. The electrode disclosed herein can be used for both the positive and the negative electrode. Test examples

[0091] The following describes test examples for the present invention. However, this should not be understood as a limitation of the present invention. 1. Test examples (1) Test example 1

[0092] Using a negative electrode in which no lithium salt was added to the mixing layer of the negative electrode, a lithium-ion secondary battery was produced in test example 1 to provide a comparative reference.

[0093] Specifically, an active material for the positive electrode (LiNi) was first developed. 1 / 3 Co 1 / 3 Mn 1 / 3 O2), an electrically conductive material (acetylene carbon black), and a binder (polyacrylic acid) were mixed in a 94:3:3 ratio and dispersed in a solvent (water) to produce a positive electrode mixture paste. This positive electrode mixture was coated on both sides of a positive electrode current collector (15 µm thick aluminum foil), followed by drying and rolling to produce a sheet-shaped positive electrode.

[0094] In this test example, a negative electrode active material (spheroidal graphite), a thickening agent (CMC), and a binder (PVdF) were mixed in a 98:1:1 ratio and dispersed in a non-salt-soluble solvent (NMP) to produce a negative electrode mixture paste. This negative electrode mixture was coated onto both sides of a negative electrode current collector (10 µm thick copper foil), followed by drying and rolling to create a sheet-shaped negative electrode.

[0095] The separator (thickness = 20 µm) used in this test example had a three-layer structure consisting of a polyethylene layer (PE layer) sandwiched between two polypropylene layers (PP layers).

[0096] After the positive electrode was stacked with the negative electrode and the separator placed between them, the resulting stack was wound to produce a flat coiled electrode assembly. The coiled electrode assembly was electrically connected via the electrode terminals (positive and negative) and then placed in a rectangular case along with the electrolyte solution. The case was then sealed to produce the lithium-ion secondary battery (battery capacity = 5 Ah) of Test Example 1. A non-aqueous electrolyte solution was used; this solution contained a conducting salt (LiPF6) at a concentration of approximately 1.0 M in a mixed solvent containing EC, DMC, and EMC in a volume ratio of 3:4:3. (2) Test examples 2 to 16

[0097] Here, the negative electrode mixing paste was produced by adding lithium salt to the paste and using a salt-dissolving solvent (water). The lithium-ion secondary batteries of test examples 2 to 16 were each produced using the same conditions as in test example 1, except that the binder was changed to CMC in accordance with the change of solvent.

[0098] For the lithium-ion secondary batteries of test examples 2 to 16, the type of lithium salt used and the amount of lithium salt added were changed according to Table 1, with only test examples 2 to 12 being covered by the present invention. (3) Test examples 17 to 20 (not covered by the present invention)

[0099] In test examples 17 to 20, lithium salt was added to the negative electrode mixture paste, and a solvent that does not dissolve salts (NMP) was used in combination with it. The lithium-ion secondary batteries were manufactured using the same conditions as in test examples 2 to 16, except that, in accordance with the change in solvent, the binder was changed to PVdF. (4) Test example 21

[0100] In test example 21, a lithium-ion secondary battery was produced in which, unlike in test examples 1 to 20, the lithium salt was added to the mixture layer of the positive electrode. Specifically, a lithium salt (lithium difluorophosphate) was added to the positive electrode mixture during the production of the positive electrode mixture paste, and a salt-dissolving solvent (water) was used for the production of the positive electrode mixture. The other conditions were the same as in test example 1. 2. Assessment tests(1) Measurement of S3 / S1

[0101] In these test examples, the ratio (S3 / S1) of the amount S3 of the lithium salt component in the third area to the amount S1 of the lithium salt component in the first area was then measured on the lithium-ion secondary batteries of test examples 1 to 21.

[0102] After the lithium-ion secondary battery was manufactured, it was first disassembled, the negative electrode removed, and the coating layer of the negative electrode divided into three equal parts along its thickness. Specifically, the following peeling process was performed three times: An adhesive tape with an adhesive force adjusted to peel off one-third of the coating layer of the negative electrode was applied to the coating layer, and then the tape was peeled off. This resulted in one adhesive tape supporting the first part of the coating layer of the negative electrode, one supporting the second part, and one supporting the third part.

[0103] Ion chromatography (IC) was performed on the mixing layer of the negative electrode of the resulting first and third regions to measure the amount S1 of the lithium salt component in the first region and the amount S3 of the lithium salt component in the third region, and S3 / S1 was calculated based on the measurement results. (2) Measurement of aggregate particle diameter

[0104] In addition, in these test examples, the particle diameter of the lithium salt aggregates was measured on the lithium-ion secondary batteries of test examples 1 to 21 by performing a cross-sectional EDS analysis.

[0105] Specifically, after fabrication, the negative electrode was removed from the lithium-ion secondary battery, and the mixture layer of this negative electrode was sectioned along its thickness direction. Using a scanning electron microscope (SEM), a photograph of the cross-section was taken, and elemental mapping of the lithium salt within the cross-section of the negative electrode's mixture layer was performed by analyzing the SEM image using energy-dispersive X-ray spectroscopy (EDS). Aggregates ≥ 0.5 µm were identified by analyzing the resulting mapping image, and the maximum particle diameter of these aggregates was measured. (3) IU resistance ratio

[0106] For each battery in test examples 1 to 21, the IU resistance ratio was also measured. Specifically, the battery of each test example was charged to 50% SOC in an environment with a temperature of 25 °C, followed by a 10-second discharge at a current of 100 Ah. The battery resistance (IU resistance) was then determined based on the discharge current and the voltage drop during discharge. The IU resistance ratio for each test example was calculated using the IU resistance value of test example 1 as a reference (100%). These results are shown in Table 1. Table 1 Test example electrode lithium salt Amount added (wt.%) S3 / S1 Aggregate particle diameter (µm) IU resistance ratio % 1 - - - - - 100 2 negative electrode Lithium difluorophosphate 0,05 1 - 80 3 negative electrode Lithium difluorophosphate 0.1 1 - 69 4 negative electrode Lithium difluorophosphate 1 0,5 - 68 5 negative electrode Lithium difluorophosphate 1 1 - 63 6 negative electrode Lithium difluorophosphate 5 1 - 79 7 negative electrode Lithium difluorophosphate 6 1 - 85 8 negative electrode Lithium difluorophosphate 10 1 - 87 9 negative electrode Lithium difluorophosphate 1 3 - 69 10 negative electrode Lithium difluorophosphate 1 4 - 72 11 negative electrode Lithium difluorophosphate 1 1 0,5 75 12 negative electrode Lithium difluorophosphate 1 1 0,8 77 13 negative electrode Lithium sulfate 1 1 - 75 14 negative electrode Lithium acetate 1 1 - 65 15 negative electrode Lithium monofluorophosphate 1 1 - 79 16 negative electrode Lithium monofluorophosphate 1 1 - 66 17 negative electrode Lithium difluorophosphate 1 0 - 85 18 negative electrode Lithium difluorophosphate 1 5 - 95 19 negative electrode Lithium difluorophosphate 1 1 1 99 20 negative electrode Lithium difluorophosphate 1 1 2 100 21 positive electrode Lithiumdifluorophosphat 1 1 - 60

[0107] As a result of comparing test example 1 with test examples 2 to 20, a reduction in IU resistance was confirmed for almost all test examples following the addition of lithium salt to the mixing layer of the negative electrode. Based on this, it was confirmed that lithium salts possessed a lithium uptake capacity and that the addition of such a lithium salt could achieve an efficient supply of lithium ions to the active material of the negative electrodes in the mixing layer of the negative electrode, thus improving the input-output properties.

[0108] However, among test examples 2 to 20, there were some, such as test examples 17 to 20, where the addition of lithium salts had little effect and an adequate reduction in IU resistance could not be achieved.

[0109] Specifically, in test example 17, where S3 / S1 = 0, and test example 18, where S3 / S1 = 5, the reduction in IU resistance was small, and no adequate improvement in the input / output characteristics could be achieved. This is interpreted as being due to the segregation of the lithium salt in certain areas of the electrode mixture layer and the inability to efficiently supply Li ions to the electrode active material across the entire electrode mixture layer.

[0110] The extent of the reduction in IU resistance was also small in test examples 19 and 20, and in particular, in test example 20, the effect of adding lithium salts was completely absent. This is assumed to be due to the following: In test examples 19 and 20, large lithium salt aggregates with a maximum particle diameter of 1 µm or larger were formed, and Li ions were attracted and concentrated to these aggregates. Consequently, Li ions could not be efficiently supplied to the electrode active material across the entire electrode mixture layer.

[0111] In contrast to the previous findings, it was confirmed that the IU resistance underwent a beneficial reduction and the input-output characteristics in test examples 2 to 16 were satisfactorily improved. Based on this, it was shown that by forming a mixture layer of the negative electrode in which the maximum particle diameter of the lithium salt aggregates is less than 1 µm and in which the relationship 0 < S3 / S1 ≤ 4 is satisfied with respect to S3 / S1, which is the ratio between the amounts of the lithium salt component in the third region A3 and the first region A1, a beneficial distribution of the lithium salt is achieved, and Li ions can be efficiently supplied to each section of the active material of the negative electrode.

[0112] In a comparison of test samples 5, 13, 14, and 15, the IU resistance showed a satisfactory decrease in all of these test samples, although the IU resistance was reduced most significantly in test sample 5. Based on this, the following could be confirmed: At least lithium acetate, lithium monofluorophosphate, and lithium difluorophosphate can be used as lithium salts for addition to the mixing layer of the negative electrode, with lithium difluorophosphate being particularly preferred.

[0113] A comparison of test examples 2 to 8 shows the following: The amount of lithium salt added has an effect on the IU resistance, and by adjusting the amount of lithium salt added to the range of 0.1 wt.% to 5 wt.%, as in test examples 3 to 6, the IU resistance is reduced more advantageously and the input / output characteristics can be improved.

[0114] The present invention has been described in detail above, but the embodiments provided above are merely examples and the invention disclosed herein comprises various modifications and alterations of the specific examples shown above.

Claims

Lithium-ion secondary battery electrode (20, 120, 220) comprising a foil-like electrode current collector (22, 122, 222), an electrode mixing layer (24, 124, 224) arranged on a surface of the electrode current collector (22, 122, 222) and containing a particulate electrode active material (23, 123, 223), wherein a lithium salt (25, 125) with a lithium ion uptake capacity is added to the electrode mixing layer (24, 124, 224), wherein the lithium salt (25, 125) is lithium difluorophosphate, the electrode mixing layer (24, 124, 224) is divided in its thickness direction into three equal regions (A1, A2, A3), and wherein from the electrode current collector side to the surface of the electrode mixing layer (24, 124, 224) these three areas are a first area (A1), a second area (A2) and a third area (A3),the amount S1 of the lithium salt component in the first region (A1) and the amount S3 of the lithium salt component in the third region (A3) satisfy the relationship in formula (1); and a maximum particle diameter of aggregates of the lithium salt (25, 125) present in the electrode mixture layer (24, 124, 224) is less than 1 µm: 0 < S3 / S1 ≤ 4 Lithium-ion secondary battery electrode (20, 120, 220) according to claim 1, wherein the surface of the electrode active material (23, 123, 223) is coated with the lithium salt (25, 125) in at least the first region (A1). Lithium-ion secondary battery electrode (20, 120, 220) according to claim 1 or 2, wherein the content of the lithium salt (25, 125) is 0.1 wt.% to 5 wt.%, with a total mass of the solid content of the electrode mixture layer (24, 124, 224) of 100 wt.%. Lithium-ion secondary battery, which is provided with a positive electrode in which a mixing layer of the positive electrode is formed on a surface of a current collector (22, 122, 222) of the positive electrode, a negative electrode in which a mixing layer of the negative electrode is formed on a surface of a current collector (22, 122, 222) of the negative electrode, and an electrolyte solution which is filled between the positive electrode and the negative electrode, wherein for at least one of the positive electrode and the negative electrode the lithium-ion secondary battery electrode (20, 120, 220) according to one of claims 1 to 3 is used. Lithium-ion secondary battery according to claim 4, wherein the lithium-ion secondary battery electrode (20, 120, 220) is used for the positive electrode.

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