Charging condition determination method and storage element

Abstract

To provide a charging condition determination method and a power storage element for determining a condition under which charging can be performed while suppressing the deposition of metallic lithium.SOLUTION: A charging condition determination method includes, in an experimental device 10, providing a three-electrode cell 11 including a positive electrode 12, a negative electrode 13, and a reference electrode 14, charging the three-electrode cell 11 by controlling the negative electrode potential with reference to the reference electrode 14, and determining a charging condition using the profile obtained by the charging operation. The reference electrode 14 has a porous portion.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a charging condition determination method and an energy storage element. [Background technology]

[0002] Due to their high energy density, non-aqueous electrolyte secondary batteries, such as lithium ion secondary batteries, are widely used in electronic devices such as personal computers and communication terminals, automobiles, etc. Non-aqueous electrolyte secondary batteries generally have a pair of electrodes electrically isolated by a separator and a non-aqueous electrolyte interposed between the electrodes, and are configured to charge and discharge by transferring ions between the electrodes. In addition to non-aqueous electrolyte secondary batteries, capacitors such as lithium ion capacitors and electric double layer capacitors are also widely used as energy storage elements.

[0003] In the case of lithium-ion secondary batteries, a material capable of absorbing and releasing lithium ions is typically used as the negative electrode active material. However, depending on the charging conditions, metallic lithium may be deposited on the surface of the negative electrode. The deposition of metallic lithium is undesirable because it can cause internal short circuits, etc. Patent Document 1 proposes a method for determining the deposition of metallic lithium on the negative electrode surface of such lithium-ion secondary batteries, in which the amount of change per unit time in battery voltage during constant current charging is detected and the deposition of metallic lithium is determined based on this amount of change. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-89363 Summary of the Invention [Problem to be solved by the invention]

[0005] On the other hand, to suppress the deposition of metallic lithium during use of an energy storage device, it is necessary to set appropriate charging conditions for the energy storage device at the design stage. Because metallic lithium deposition is particularly likely to occur during rapid charging or low-temperature charging, it is desirable to develop a method for determining the conditions under which rapid charging or low-temperature charging can be performed while suppressing metallic lithium deposition. To achieve this, it is important to be able to measure the negative electrode potential with high accuracy.

[0006] The present invention has been made based on the above circumstances, and an object of the present invention is to provide a method for determining conditions under which charging can be performed while suppressing the deposition of metallic lithium, and an energy storage element capable of measuring the negative electrode potential with high accuracy. [Means for solving the problem]

[0007] A method for determining charging conditions according to one aspect of the present invention includes preparing a three-electrode cell including a positive electrode, a negative electrode, and a reference electrode; charging the three-electrode cell by controlling the negative electrode potential based on the reference electrode; and determining charging conditions using a profile obtained by the charging operation, wherein the reference electrode has a porous portion.

[0008] An electric storage device according to another aspect of the present invention includes a positive electrode, a negative electrode, and a reference electrode, and the reference electrode has a porous portion with a porosity of 50% or more. [Effects of the Invention]

[0009] According to one aspect of the present invention, it is possible to provide a method for determining conditions under which charging can be performed while suppressing the deposition of metallic lithium, and an energy storage device capable of measuring the negative electrode potential with high accuracy. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic diagram showing an experimental device used in one embodiment of the charging condition determining method. [Figure 2]FIG. 2 is a schematic cross-sectional view showing a reference electrode used in one embodiment of the method for determining charging conditions. [Figure 3] FIG. 3 is a graph showing the results of a simulation of the transition of the negative electrode potential relative to the reference electrode during constant current charging at a current of 10 C. [Figure 4] FIG. 4 is a graph showing the results of a simulation of the transition of the negative electrode potential relative to the reference electrode during constant current charging at a current of 20 C. [Figure 5] FIG. 5 is a graph showing an example of a profile (changes in negative electrode potential and current relative to the reference electrode) obtained when constant-current constant-potential charging is performed as a charging operation, as one embodiment of a method for determining charging conditions. [Figure 6] FIG. 6 is a perspective view showing an embodiment of an energy storage element. [Figure 7] FIG. 7 is a schematic diagram showing an embodiment of an electricity storage device configured by assembling a plurality of electricity storage elements. DETAILED DESCRIPTION OF THE INVENTION

[0011] First, an outline of the charging condition determination method and the energy storage element disclosed in this specification will be described.

[0012] A method for determining charging conditions according to one aspect of the present invention includes preparing a three-electrode cell including a positive electrode, a negative electrode, and a reference electrode; charging the three-electrode cell by controlling the negative electrode potential based on the reference electrode; and determining charging conditions using a profile obtained by the charging operation, wherein the reference electrode has a porous portion.

[0013] According to the method for determining charging conditions, it is possible to determine conditions under which charging can be performed while suppressing the deposition of metallic lithium for the following reasons. The deposition of metallic lithium on the negative electrode thermodynamically occurs when the negative electrode potential is 0 V (vs. Li / Li +) or less. Therefore, to prevent metallic lithium deposition, it is necessary to accurately grasp the negative electrode potential. In this case, a reference electrode is typically used to measure the negative electrode potential based on the reference electrode. However, when a conventional non-porous reference electrode is placed between the positive and negative electrodes, it inhibits the movement of lithium ions in the electrolyte, affecting the measurement results. If the reference electrode is placed somewhere other than between the positive and negative electrodes, such as on the back side of the negative electrode (the side opposite the surface facing the positive electrode) to avoid inhibiting the movement of lithium ions, an accurate measurement of the negative electrode potential is impossible. In contrast, the charging condition determination method uses a reference electrode with a porous portion, so that the movement of lithium ions is less inhibited even when the reference electrode is placed between the positive and negative electrodes. Therefore, to accurately grasp the negative electrode potential, it is necessary to essentially consider only the IR drop (voltage drop due to the resistance of the electrolyte, etc.) between the negative electrode and the reference electrode, enabling highly accurate measurement of the negative electrode potential. Therefore, by using such a reference electrode and determining the charging conditions using a profile obtained by a charging operation in which the negative electrode potential is controlled based on this reference electrode, it is possible to determine the conditions under which charging can be performed while suppressing the deposition of metallic lithium.

[0014] Here, "porous" means that the porosity is greater than 10%. In other words, a porous site is a site where the porosity is greater than 10%. Furthermore, the "porosity" is a value calculated by the following formula 1. Porosity (%) = {1 - (bulk density / theoretical density)} × 100 1 Bulk density is the value obtained by dividing the mass of the portion to be measured (porous portion) by the volume. Theoretical density is the density (true density) of the material itself that constitutes the portion to be measured. The volume of the portion to be measured can be determined from the area determined by image processing of an image acquired by a digital camera or electron microscope, and the thickness measured with a micrometer. For the reference electrode provided in the energy storage element described below, the energy storage element is disassembled, the reference electrode is removed, and the electrode is thoroughly washed with dimethyl carbonate and dried under reduced pressure at room temperature, and then the measurement is performed.

[0015] The reference electrode preferably has a wire made of a metal that does not alloy with lithium, the entire surface of which is covered with an insulating layer, and the porous portion is disposed so as to cover the tip of the wire made of a metal that does not alloy with lithium. By using such a reference electrode, the negative electrode potential can be measured with higher accuracy, and the reference electrode is relatively easy to manufacture. Examples of the wire made of a metal that does not alloy with lithium include copper wire, SUS wire, and cobalt wire. Among these, copper wire is preferred from the viewpoint of cost.

[0016] The charging operation preferably includes constant-current, constant-potential charging. By using a profile obtained by constant-current, constant-potential charging, it is possible to efficiently determine charging conditions that are less likely to cause deposition of metallic lithium.

[0017] The porosity of the porous portion is preferably 50% or more. By using a reference electrode having a porous portion with such high porosity, the negative electrode potential can be measured with higher accuracy, and as a result, charging conditions that are less likely to cause deposition of metallic lithium can be determined more accurately.

[0018] An electric storage device according to another aspect of the present invention includes a positive electrode, a negative electrode, and a reference electrode, and the reference electrode has a porous portion with a porosity of 50% or more.

[0019] The energy storage element includes a reference electrode having a porous portion with a porosity of 50% or more, and therefore, the negative electrode potential can be measured with high accuracy. Furthermore, the energy storage element also allows the positive electrode potential to be measured with high accuracy.

[0020] A method for determining charging conditions, a storage element, a storage device, a method for manufacturing a storage element, and other embodiments according to one embodiment of the present invention will be described in detail. Note that the names of the components used in each embodiment may differ from the names of the components used in the background art.

[0021] <How to determine charging conditions> A method for determining charging conditions according to one embodiment of the present invention includes preparing a three-electrode cell including a positive electrode, a negative electrode, and a reference electrode, performing a charging operation on the three-electrode cell by controlling the negative electrode potential based on the reference electrode, and determining charging conditions using a profile obtained by the charging operation.

[0022] FIG. 1 shows an experimental apparatus 10 including a three-electrode cell 11. The three-electrode cell 11 includes a positive electrode 12, a negative electrode 13, and a reference electrode 14. The positive electrode 12, the negative electrode 13, and the reference electrode 14 are immersed in an electrolyte 15. The three-electrode cell 11 may further include a separator (not shown) disposed between each electrode. The three-electrode cell 11 is connected to a control PC (personal computer) 17 via a charge / discharge device 16. The control PC 17 controls the charge / discharge device 16 according to a charge / discharge program stored in its memory. The three-electrode cell 11 is charged and discharged by the charge / discharge device 16.

[0023] The positive electrode 12, the negative electrode 13, and the reference electrode 14 are connected to a charge / discharge device 16. The positive electrode 12 and the negative electrode 13 are not particularly limited, and conventionally known positive electrodes and negative electrodes can be used. In the case of a lithium ion secondary battery, an active material containing lithium is typically used as the active material of at least one of the positive electrode 12 and the negative electrode 13. Specific examples of the positive electrode and the negative electrode include the positive electrode and the negative electrode provided in the energy storage element according to one embodiment of the present invention described below. The reference electrode 14 will be described in detail later. Furthermore, conventionally known electrolytes can be used for the electrolyte 15. In the case of a lithium ion secondary battery, the electrolyte 15 can be a nonaqueous electrolyte solution containing a lithium salt and a nonaqueous solvent. Specific examples of the electrolyte 15 include the electrolyte provided in the energy storage element according to one embodiment of the present invention described below. The control PC 17 detects the potential of the negative electrode 13 relative to the reference electrode 14 via the charge / discharge device 16. The control PC 17 may also be capable of detecting the potential of the positive electrode 12 relative to the reference electrode 14.

[0024] In the three-electrode cell 11 of Fig. 1, the reference electrode 14 is disposed between the positive electrode 12 and the negative electrode 13. The reference electrode 14 has a porous portion. Fig. 2 shows a preferred form of the reference electrode 14.

[0025] The reference electrode 14 in Fig. 2 has a copper wire 18 that is covered with an insulating layer 19 except for a tip portion 18a. In the reference electrode 14 in Fig. 2, the tip portion 18a that is not covered with the insulating layer 19 is the end face of the copper wire 18. A porous portion 20 is arranged so as to cover the tip portion 18a (end face) of the copper wire 18.

[0026] The material for forming the insulating layer 19 is not particularly limited as long as it has insulating properties, and for example, resin or the like can be used. Insulating properties refer to non-conductivity. Whether or not a material has "conductivity" is determined by a volume resistivity of 10 as measured in accordance with JIS-H-0505 (1975). 7 The resistance is determined using Ω·cm as a threshold value. The diameter of the copper wire 18 coated with the insulating layer 19 (the diameter including the thickness of the insulating layer 19) is, for example, approximately 10 μm or more and 100 μm or less. In addition, using the copper wire 18 for the reference electrode 14, i.e., the core material of the reference electrode 14 is metallic copper, has the advantage that when the porous portion 20 is metallic lithium, the metallic lithium and metallic copper do not alloy with each other, and the potential relative to metallic lithium can be obtained with higher accuracy.

[0027] The porous region 20 is a portion that serves as a reference for the potential of the reference electrode 14. The material of the porous region 20 is not particularly limited as long as it does not substantially affect the charge / discharge performance of the three-electrode cell 11 and allows the potential difference with the negative electrode 13 to be measured, but metallic lithium is preferred. The metallic lithium may be pure metallic lithium consisting of only lithium element, or may be a lithium alloy containing other elements.

[0028] The length L of the porous region 20 can be, for example, 500 μm or more and 10 mm or less. The thickness T of the porous region 20 can be, for example, 5 μm or more and 100 μm or less. The thickness T of the porous region 20 can be, for example, approximately the same as the diameter of the copper wire 18 covered with the insulating layer 19, or it can be greater than that. The porous region 20 can have a flat plate shape. The flat plate-shaped porous region 20 can have, for example, a substantially circular shape centered on the tip end 18a of the copper wire 18 in a planar view, or a substantially rectangular shape. The width of the porous region 20 in a planar view (the maximum length in a direction perpendicular to the length L in a planar view) can be, for example, 500 μm or more and 10 mm or less.

[0029] When the porous region 20 is metallic lithium, the porous region 20 can be formed, for example, in a three-electrode cell 11, by passing a current from a reference electrode 14 that does not have a porous region 20 to a positive electrode 12 (performing a charging operation between the positive electrode and the reference electrode). By performing this operation, lithium ions typically contained in the electrolyte are deposited as metallic lithium on the tip 18a of the copper wire 18, forming the porous region 20 made of metallic lithium. Note that an electrode assembly can be formed by placing a reference electrode 14 that does not have a porous region 20 (a copper wire 18 whose tip 18a is not covered with an insulating layer 19) between a plate-shaped positive electrode 12 and a plate-shaped negative electrode 13, with a separator interposed therebetween, and performing the charging operation described above while compressing the electrode assembly, thereby forming a flat-plate-shaped porous region 20 made of metallic lithium.

[0030] The lower limit of the porosity of the porous region 20 may be, for example, 30%, but is preferably 50%, more preferably 60%, and even more preferably 70%. Increasing the porosity of the porous region 20 can improve the accuracy of the measured negative electrode potential. On the other hand, from the viewpoint of the strength of the porous region 20, the porosity of the porous region 20 is preferably, for example, 90% or less, and more preferably 80% or less in some cases. The porosity of the porous region 20 can be adjusted, for example, by changing the current density, time, temperature, distance between the positive electrode and the reference electrode, etc. in the above-mentioned charging operation.

[0031] Regarding the influence of the porosity of the porous region 20, Figures 3 and 4 show simulation results of the transition of the negative electrode potential relative to the reference electrode during constant-current charging at 10 C or 20 C. Each simulation shows the results when the porosity of the metallic lithium serving as the porous region in the reference electrode is assumed to be 1% (non-porous), 10% (non-porous), 50% (porous), and 100%, respectively. A porosity of 100% assumes no interference from the reference electrode. Figure 3 shows the results when the charging current is 10 C, and Figure 4 shows the results when the charging current is 20 C. These simulations are based on a model in which the reference electrode is placed between the positive and negative electrodes. The negative electrode potential relative to the reference electrode is measured as a value theoretically reduced by the IR drop from the true negative electrode potential. The IR drop can be calculated from the set current I and the resistance R obtained from impedance measurements, etc. The negative electrode potential measured relative to a 100% porosity reference electrode is the negative electrode potential measured by the IR drop relative to the true negative electrode potential, and is theoretically the most accurate negative electrode potential. As shown in Figures 3 and 4, the negative electrode potential measured relative to a 50% porosity reference electrode is nearly identical to the negative electrode potential measured relative to a 100% porosity reference electrode. This indicates that the true negative electrode potential can be accurately determined by correcting for the IR drop. On the other hand, the negative electrode potential measured relative to 1% and 10% porosity reference electrodes differs significantly from the negative electrode potential measured relative to a 100% porosity reference electrode, particularly in Figure 4, where the current is large. This indicates that the negative electrode potential cannot be accurately measured. Therefore, the true negative electrode potential cannot be accurately determined by simply correcting for the IR drop.

[0032] Next, we will explain the charging operation for obtaining a profile (data on the transition of current, etc. during charging). The charging operation is performed by controlling the negative electrode potential with respect to the reference electrode for the three-electrode cell. This charging operation can be performed, for example, as constant-current constant-potential charging. An example of constant-current constant-potential charging is shown in Figure 5. In this example, the negative electrode potential is set to 0 V (vs. Li / Li +) was charged at a constant current until the negative electrode potential reached 0 V (vs. Li / Li + ), charging is performed while controlling so that this negative electrode potential is maintained.

[0033] The charging conditions can be determined using a profile obtained by such a charging operation. For example, multi-stage charging conditions can be set based on the results of Figure 5. Specifically, by setting the charging conditions to charge at current A when the amount of charge is between 0 and a in Figure 5, current B when the amount of charge is between a and b, current C when the amount of charge is between b and c, current D when the amount of charge is between c and d, and current E when the amount of charge is between d and e, the negative electrode potential can be set to 0 V (vs. Li / Li + ), that is, charging conditions under which deposition of metallic lithium is suppressed can be obtained.

[0034] For example, when constant current constant potential charging is performed, the negative electrode potential during constant potential charging is 0 V (vs. Li / Li + For example, the negative electrode potential during constant-potential charging may be controlled to a predetermined value, since the overpotential generated varies depending on the current density, etc., and the potential at which metallic lithium deposition occurs varies. In addition, the charging conditions may be determined by adding a correction for the IR drop to the obtained profile.

[0035] Furthermore, the charging operation for obtaining the profile is not limited to single-stage constant-current constant-potential charging, but may employ multi-stage constant-current charging, multi-stage constant-current constant-potential charging, etc. When multi-stage constant-current charging, etc. is performed, the termination condition for charging at each current value is, for example, when the negative electrode potential reaches 0 V (vs. Li / Li + ) or a predetermined potential such as 0 V. The negative electrode potential at the end of charge in each stage may be the same or different. When a profile is obtained by multi-stage constant current charging or the like, charging conditions can be determined based on the obtained profile in the same way as when the profile obtained by constant current / constant potential charging is used.

[0036] A method for determining charging conditions according to one embodiment of the present invention can be suitably employed as a method for determining multi-stage charging conditions for rapid charging or low-temperature charging. Rapid charging refers to charging with a current whose maximum value is 2 C or more, and the maximum value of the current may be 10 C or more or 20 C or more. The upper limit of the maximum current may be, for example, 100 C. Low-temperature charging refers to charging in an environment of 0°C or below, and the current value in this case is not limited to the above.

[0037] The above-described rapid charging and low-temperature charging can be employed in, for example, automotive power sources (power storage elements) for electric vehicles (EVs), plug-in hybrid vehicles (PHEVs), hybrid vehicles (HEVs), etc., industrial power sources (power storage elements), stationary power sources (power storage elements), etc. Therefore, the charging condition determination method according to one embodiment of the present invention can be suitably used as a charging condition determination method for automotive power sources (power storage elements), industrial power sources (power storage elements), stationary power sources (power storage elements), etc.

[0038] <Energy storage element> An energy storage element according to one embodiment of the present invention comprises an electrode assembly having a positive electrode, a negative electrode, and a separator, an electrolyte, and a container for accommodating the electrode assembly and the electrolyte. The electrode assembly is typically a stacked type in which a plurality of positive electrodes and a plurality of negative electrodes are stacked with separators interposed therebetween, or a wound type in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween and wound. The electrolyte is present in a state of being impregnated in the positive electrode, the negative electrode, and the separator. The energy storage element further comprises a reference electrode. As an example of an energy storage element, a nonaqueous electrolyte secondary battery (hereinafter also simply referred to as a "secondary battery") will be described.

[0039] (positive electrode) The positive electrode has a positive electrode substrate and a positive electrode active material layer disposed on the positive electrode substrate directly or via an intermediate layer.

[0040] The positive electrode substrate is conductive. Examples of the material for the positive electrode substrate include metals such as aluminum, titanium, tantalum, and stainless steel, as well as alloys thereof. Among these, aluminum or aluminum alloys are preferred from the viewpoints of potential resistance, high conductivity, and cost. Examples of the positive electrode substrate include foil, vapor-deposited film, mesh, and porous material, with foil being preferred from the viewpoint of cost. Therefore, aluminum foil or aluminum alloy foil is preferred as the positive electrode substrate. Examples of aluminum or aluminum alloys include A1085, A3003, and A1N30, as specified in JIS-H-4000 (2014) or JIS-H4160 (2006).

[0041] The average thickness of the positive electrode substrate is preferably 3 μm to 50 μm, more preferably 5 μm to 40 μm, even more preferably 8 μm to 30 μm, and particularly preferably 10 μm to 25 μm. By setting the average thickness of the positive electrode substrate within the above range, the strength of the positive electrode substrate can be increased while increasing the energy density per volume of the energy storage element.

[0042] The intermediate layer is a layer disposed between the positive electrode substrate and the positive electrode active material layer. The intermediate layer contains a conductive agent such as carbon particles to reduce the contact resistance between the positive electrode substrate and the positive electrode active material layer. The configuration of the intermediate layer is not particularly limited, and may contain, for example, a binder and a conductive agent.

[0043] The positive electrode active material layer contains a positive electrode active material and, if necessary, optional components such as a conductive agent, a binder, a thickener, and a filler.

[0044] The positive electrode active material can be appropriately selected from known positive electrode active materials. A material capable of absorbing and releasing lithium ions is usually used as the positive electrode active material for a lithium ion secondary battery. Examples of the positive electrode active material include lithium transition metal composite oxides having an α-NaFeO2 crystal structure, lithium transition metal composite oxides having a spinel crystal structure, polyanion compounds, chalcogen compounds, and sulfur. Examples of lithium transition metal composite oxides having an α-NaFeO2 crystal structure include Li[Lix Ni (1-x) ]O2(0≦x<0.5), Li[Li x Ni γ Co (1-x-γ) ]O2(0≦x<0.5, 0<γ<1, 0<1-x-γ), Li[Li x Co (1-x) ]O2(0≦x<0.5), Li[Li x Ni γ Mn (1-x-γ) ]O2(0≦x<0.5, 0<γ<1, 0<1-x-γ), Li[Li x Ni γ Mn β Co (1-x-γ-β) ]O2(0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1, 0<1-x-γ-β), Li[Li x Ni γ Co β Al (1-x-γ-β) ]O2 (0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1, 0<1-x-γ-β). Lithium transition metal composite oxides with spinel-type crystal structures include Li x Mn2O4, Li x Ni γ Mn (2-γ) Examples of polyanion compounds include LiFePO4, LiMnPO4, LiNiPO4, LiCoPO4, Li3V2(PO4)3, Li2MnSiO4, and Li2CoPO4F. Examples of chalcogen compounds include titanium disulfide, molybdenum disulfide, and molybdenum dioxide. Atoms or polyanions in these materials may be partially substituted with atoms or anion species of other elements. The surfaces of these materials may be coated with other materials. In the positive electrode active material layer, one of these materials may be used alone, or two or more may be used in combination.

[0045] The positive electrode active material is usually in the form of particles (powder). The average particle size of the positive electrode active material is preferably, for example, 0.1 μm or more and 20 μm or less. By setting the average particle size of the positive electrode active material to the above lower limit or more, the positive electrode active material becomes easier to manufacture and handle. By setting the average particle size of the positive electrode active material to the above upper limit or less, the electronic conductivity of the positive electrode active material layer is improved. Note that when a composite of the positive electrode active material and another material is used, the average particle size of the composite is taken as the average particle size of the positive electrode active material. The "average particle size" refers to the value at which the volume-based cumulative distribution calculated in accordance with JIS-Z-8819-2 (2001) is 50% based on the particle size distribution measured by laser diffraction / scattering in a diluted solution obtained by diluting particles with a solvent in accordance with JIS-Z-8825 (2013).

[0046] To obtain powders with a predetermined particle size, grinders, classifiers, etc. are used. Grinding methods include, for example, methods using a mortar, ball mill, sand mill, vibration ball mill, planetary ball mill, jet mill, counter jet mill, swirling airflow jet mill, or sieves. Wet grinding in the presence of water or an organic solvent such as hexane can also be used during grinding. As classification methods, sieves, air classifiers, etc. are used as needed for both dry and wet methods.

[0047] The content of the positive electrode active material in the positive electrode active material layer is preferably 50% by mass to 99% by mass, more preferably 70% by mass to 98% by mass, and even more preferably 80% by mass to 95% by mass. By setting the content of the positive electrode active material within this range, both high energy density and manufacturability of the positive electrode active material layer can be achieved.

[0048] The conductive agent is not particularly limited as long as it is a material having electrical conductivity. Examples of such conductive agents include carbonaceous materials, metals, and conductive ceramics. Examples of carbonaceous materials include graphite, non-graphitic carbon, and graphene-based carbon. Examples of non-graphitic carbon include carbon nanofibers, pitch-based carbon fibers, and carbon black. Examples of carbon black include furnace black, acetylene black, and ketjen black. Examples of graphene-based carbon include graphene, carbon nanotubes (CNTs), and fullerenes. The conductive agent may be in the form of powder or fiber. As the conductive agent, one of these materials may be used alone, or two or more may be mixed. These materials may also be used in combination. For example, a composite of carbon black and CNTs may be used. Among these, carbon black is preferred from the viewpoints of electronic conductivity and coatability, and acetylene black is particularly preferred.

[0049] The content of the conductive agent in the positive electrode active material layer is preferably 1% by mass to 10% by mass, more preferably 3% by mass to 9% by mass. By setting the content of the conductive agent within this range, the energy density of the energy storage element can be increased.

[0050] Examples of binders include thermoplastic resins such as fluororesins (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc.), polyethylene, polypropylene, polyacrylic, and polyimide; elastomers such as ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), and fluororubber; and polysaccharide polymers.

[0051] The binder content in the positive electrode active material layer is preferably 1% by mass to 10% by mass, more preferably 3% by mass to 9% by mass. By setting the binder content within this range, the active material can be stably maintained.

[0052] Examples of thickeners include polysaccharide polymers such as carboxymethyl cellulose (CMC), methyl cellulose, etc. When the thickener has a functional group that reacts with lithium or the like, this functional group may be deactivated in advance by methylation or the like.

[0053] The filler is not particularly limited, and examples of the filler include polyolefins such as polypropylene and polyethylene, inorganic oxides such as silicon dioxide, alumina, titanium dioxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicates, hydroxides such as magnesium hydroxide, calcium hydroxide, and aluminum hydroxide, carbonates such as calcium carbonate, sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium sulfate, nitrides such as aluminum nitride and silicon nitride, substances derived from mineral resources such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, and artificial products thereof.

[0054] The positive electrode active material layer may contain typical non-metallic elements such as B, N, P, F, Cl, Br, and I; typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, and Ba; and transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Nb, and W as components other than the positive electrode active material, conductive agent, binder, thickener, and filler.

[0055] (Negative electrode) The negative electrode has a negative electrode substrate and a negative electrode active material layer disposed on the negative electrode substrate directly or via an intermediate layer. The configuration of the intermediate layer is not particularly limited and can be selected from the configurations exemplified for the positive electrode above, for example.

[0056] The negative electrode substrate is conductive. Metals such as copper, nickel, stainless steel, nickel-plated steel, and aluminum, alloys thereof, and carbonaceous materials are used as the material for the negative electrode substrate. Among these, copper or copper alloys are preferred. Examples of the negative electrode substrate include foils, vapor-deposited films, meshes, and porous materials, with foils being preferred from the viewpoint of cost. Therefore, copper foil or copper alloy foil is preferred as the negative electrode substrate. Examples of copper foil include rolled copper foil and electrolytic copper foil.

[0057] The average thickness of the negative electrode substrate is preferably 2 μm to 35 μm, more preferably 3 μm to 30 μm, even more preferably 4 μm to 25 μm, and particularly preferably 5 μm to 20 μm. By setting the average thickness of the negative electrode substrate within the above range, the strength of the negative electrode substrate can be increased while increasing the energy density per volume of the energy storage element.

[0058] The negative electrode active material layer contains a negative electrode active material. The negative electrode active material layer contains optional components such as a conductive agent, a binder, a thickener, and a filler, as needed. The optional components such as the conductive agent, the binder, the thickener, and the filler can be selected from the materials exemplified for the positive electrode above.

[0059] The negative electrode active material layer may contain typical non-metallic elements such as B, N, P, F, Cl, Br, and I; typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, and Ba; and transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Ta, Hf, Nb, and W as components other than the negative electrode active material, conductive agent, binder, thickener, and filler.

[0060] The negative electrode active material can be appropriately selected from known negative electrode active materials. Materials capable of absorbing and releasing lithium ions are typically used as negative electrode active materials for lithium ion secondary batteries. Examples of negative electrode active materials include metals or semimetals such as Si and Sn; metal oxides or semimetal oxides such as Si oxide, Ti oxide, and Sn oxide; and Li4Ti5O 12 , LiTiO 2、Examples of the material include titanium-containing oxides such as TiNbO; polyphosphate compounds; silicon carbide; and carbon materials such as graphite and non-graphitic carbon (easily graphitizable carbon or non-graphitizable carbon). Among these materials, graphite and non-graphitic carbon are preferred. In the negative electrode active material layer, one of these materials may be used alone, or two or more may be used in combination.

[0061] "Graphite" refers to graphite that has an average lattice spacing (d 002 ) is 0.33 nm or more and less than 0.34 nm. Examples of graphite include natural graphite and artificial graphite. Artificial graphite is preferred from the viewpoint of being able to obtain a material with stable physical properties.

[0062] "Non-graphitic carbon" refers to the carbon that has an average lattice spacing (d 002 ) is 0.34 nm or more and 0.42 nm or less. Non-graphitic carbon includes non-graphitizable carbon and graphitizable carbon. Examples of non-graphitic carbon include resin-derived materials, petroleum pitch or petroleum pitch-derived materials, petroleum coke or petroleum coke-derived materials, plant-derived materials, and alcohol-derived materials.

[0063] Here, the "discharged state" refers to a state in which the negative electrode active material, a carbonaceous material, is discharged so that lithium ions capable of being absorbed and desorbed during charging and discharging are sufficiently released. For example, this refers to a state in which the open circuit voltage of a half cell using a negative electrode containing a carbonaceous material as a negative electrode active material as a working electrode and metallic Li as a counter electrode is 0.7 V or higher.

[0064] "Non-graphitizable carbon" means the above d 002 This refers to carbon materials with a particle size of 0.36 nm or more and 0.42 nm or less.

[0065] "Graphitizable carbon" means the above d 002 This refers to carbon materials with a particle size of 0.34 nm or more and less than 0.36 nm.

[0066] The negative electrode active material is usually in the form of particles (powder). The average particle size of the negative electrode active material can be, for example, 1 nm or more and 100 μm or less. When the negative electrode active material is a carbon material, a titanium-containing oxide, or a polyphosphate compound, the average particle size may be 1 μm or more and 100 μm or less. When the negative electrode active material is Si, Sn, Si oxide, Sn oxide, or the like, the average particle size may be 1 nm or more and 1 μm or less. By setting the average particle size of the negative electrode active material to be equal to or greater than the above-mentioned lower limit, the negative electrode active material can be easily produced or handled. By setting the average particle size of the negative electrode active material to be equal to or less than the above-mentioned upper limit, the electronic conductivity of the active material layer is improved. To obtain powder with a predetermined particle size, a pulverizer, a classifier, or the like is used. The pulverization method and the powder classification method can be selected from, for example, the methods exemplified for the positive electrode. When the negative electrode active material is a metal such as Si or Sn, the negative electrode active material may be in the form of a foil.

[0067] The content of the negative electrode active material in the negative electrode active material layer is preferably 60% by mass to 99% by mass, more preferably 90% by mass to 98% by mass. By setting the content of the negative electrode active material within this range, both high energy density and manufacturability of the negative electrode active material layer can be achieved.

[0068] (Reference pole) The reference electrode has a porous portion with a porosity of 50% or more. A specific and preferred embodiment of the reference electrode provided in the energy storage element is the reference electrode described as the reference electrode used in the method for determining charging conditions according to one embodiment of the present invention, in which the porosity of the porous portion is 50% or more.

[0069] (separator) The separator can be appropriately selected from known separators. Examples of separators that can be used include separators consisting of only a substrate layer and separators in which a heat-resistant layer containing heat-resistant particles and a binder is formed on one or both surfaces of the substrate layer. Examples of the shape of the substrate layer of the separator include woven fabric, nonwoven fabric, and porous resin film. Among these shapes, porous resin film is preferred from the viewpoint of strength, and nonwoven fabric is preferred from the viewpoint of electrolyte (electrolytic solution) retention. Materials for the substrate layer of the separator are preferably polyolefins such as polyethylene and polypropylene from the viewpoint of shutdown function, and polyimide and aramid from the viewpoint of oxidative decomposition resistance. A composite material of these resins may also be used for the substrate layer of the separator.

[0070] The heat-resistant particles contained in the heat-resistant layer preferably exhibit a mass loss of 5% or less when heated from room temperature to 500°C in an air atmosphere at 1 atmosphere pressure, and more preferably exhibit a mass loss of 5% or less when heated from room temperature to 800°C. Examples of materials exhibiting a mass loss of a predetermined value or less include inorganic compounds. Examples of inorganic compounds include oxides such as iron oxide, silicon oxide, aluminum oxide, titanium oxide, zirconium oxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicate; nitrides such as aluminum nitride and silicon nitride; carbonates such as calcium carbonate; sulfates such as barium sulfate; sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium titanate; covalently bonded crystals such as silicon and diamond; mineral-derived substances such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, as well as artificial products thereof. As the inorganic compound, these substances may be used alone or in the form of a complex, or two or more of them may be used in combination. Among these inorganic compounds, silicon oxide, aluminum oxide, or aluminosilicate is preferred from the viewpoint of the safety of the energy storage device.

[0071] The porosity of the separator is preferably 80% by volume or less from the viewpoint of strength, and is preferably 20% by volume or more from the viewpoint of discharge performance. Here, "porosity" refers to a volume-based value measured with a mercury porosimeter.

[0072] The separator may be a polymer gel composed of a polymer and a non-aqueous electrolyte. Examples of polymers include polyacrylonitrile, polyethylene oxide, polypropylene oxide, polymethyl methacrylate, polyvinyl acetate, polyvinylpyrrolidone, and polyvinylidene fluoride. The use of a polymer gel has the effect of suppressing leakage. The separator may be a combination of the porous resin film or nonwoven fabric described above and a polymer gel.

[0073] (electrolyte) A nonaqueous electrolyte secondary battery uses a nonaqueous electrolyte as an electrolyte. The nonaqueous electrolyte can be appropriately selected from known nonaqueous electrolytes. A nonaqueous electrolyte solution may be used as the nonaqueous electrolyte. The nonaqueous electrolyte solution includes a nonaqueous solvent and an electrolyte salt dissolved in the nonaqueous solvent.

[0074] The non-aqueous solvent can be appropriately selected from known non-aqueous solvents. Examples of non-aqueous solvents include cyclic carbonates, chain carbonates, carboxylic acid esters, phosphate esters, sulfonic acid esters, ethers, amides, and nitriles. Non-aqueous solvents in which some of the hydrogen atoms contained in these compounds have been substituted with halogens may also be used.

[0075] Examples of cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), vinylethylene carbonate (VEC), chloroethylene carbonate, fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), styrene carbonate, 1-phenylvinylene carbonate, 1,2-diphenylvinylene carbonate, etc. Among these, EC is preferred.

[0076] Examples of the chain carbonate include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diphenyl carbonate, trifluoroethyl methyl carbonate, bis(trifluoroethyl) carbonate, etc. Among these, EMC is preferred.

[0077] It is preferable to use a cyclic carbonate or a chain carbonate as the non-aqueous solvent, and it is more preferable to use a combination of a cyclic carbonate and a chain carbonate. The use of a cyclic carbonate can promote dissociation of the electrolyte salt and improve the ionic conductivity of the non-aqueous electrolyte. The use of a chain carbonate can reduce the viscosity of the non-aqueous electrolyte. When a cyclic carbonate and a chain carbonate are used in combination, the volume ratio of the cyclic carbonate to the chain carbonate (cyclic carbonate:chain carbonate) is preferably in the range of, for example, 5:95 to 50:50.

[0078] The electrolyte salt can be appropriately selected from known electrolyte salts, such as lithium salts, sodium salts, potassium salts, magnesium salts, and onium salts. Of these, lithium salts are preferred.

[0079] Examples of lithium salts include inorganic lithium salts such as LiPF6, LiPO2F2, LiBF4, LiClO4, and LiN(SO2F)2, lithium oxalates such as lithium bis(oxalate)borate (LiBOB), lithium difluorooxalateborate (LiFOB), and lithium bis(oxalate)difluorophosphate (LiFOP), and lithium salts having a halogenated hydrocarbon group such as LiSO3CF3, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiN(SO2CF3)(SO2C4F9), LiC(SO2CF3)3, and LiC(SO2C2F5)3. Among these, inorganic lithium salts are preferred, and LiPF6 is more preferred.

[0080] The content of electrolyte salt in the non-aqueous electrolyte is 0.1 mol / dm at 20°C and 1 atmosphere.3 More than 2.5mol / dm 3 It is preferable that the value is 0.3 mol / dm or less. 3 More than 2.0mol / dm 3 It is more preferable that it is 0.5 mol / dm or less. 3 More than 1.7mol / dm 3 It is more preferable that it is 0.7 mol / dm or less. 3 More than 1.5mol / dm 3 By setting the content of the electrolyte salt within the above range, the ionic conductivity of the non-aqueous electrolyte can be increased.

[0081] The non-aqueous electrolyte may contain additives in addition to the non-aqueous solvent and the electrolyte salt. Examples of the additives include halogenated carbonates such as fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC); oxalates such as lithium bis(oxalato)borate (LiBOB), lithium difluorooxalatoborate (LiFOB), and lithium bis(oxalato)difluorophosphate (LiFOP); imide salts such as lithium bis(fluorosulfonyl)imide (LiFSI); biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, and cyclohexyl. Aromatic compounds such as benzene, t-butylbenzene, t-amylbenzene, diphenyl ether, and dibenzofuran; partial halides of the above aromatic compounds such as 2-fluorobiphenyl, o-cyclohexylfluorobenzene, and p-cyclohexylfluorobenzene; halogenated anisole compounds such as 2,4-difluoroanisole, 2,5-difluoroanisole, 2,6-difluoroanisole, and 3,5-difluoroanisole; vinylene carbonate, methyl vinylene carbonate, ethyl vinylene carbonate, succinic anhydride, Glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, cyclohexanedicarboxylic anhydride; ethylene sulfite, propylene sulfite, dimethyl sulfite, propane sultone, propene sultone, butane sultone, methyl methanesulfonate, busulfan, methyl toluenesulfonate, dimethyl sulfate, ethylene sulfate, sulfolane, dimethyl sulfone, diethyl sulfone, dimethyl sulfoxide, diethyl sulfoxide, tetramethylene sulfoxide, diphenyl sulfide, 4,4'-bis(2,2- dioxo-1,3,2-dioxathiolane), 4-methylsulfonyloxymethyl-2,2-dioxo-1,3,2-dioxathiolane, thioanisole, diphenyl disulfide, dipyridinium disulfide, 1,3-propene sultone, 1,3-propane sultone, 1,4-butane sultone, 1,4-butene sultone, perfluorooctane, tristrimethylsilyl borate, tristrimethylsilyl phosphate, tetrakistrimethylsilyl titanate, lithium monofluorophosphate, lithium difluorophosphate, and the like.These additives may be used alone or in combination of two or more.

[0082] The content of the additive contained in the non-aqueous electrolyte is preferably 0.01% by mass to 10% by mass, more preferably 0.1% by mass to 7% by mass, even more preferably 0.2% by mass to 5% by mass, and particularly preferably 0.3% by mass to 3% by mass. By setting the content of the additive within the above range, it is possible to improve the capacity retention performance or cycle performance after high-temperature storage, and further improve safety.

[0083] The non-aqueous electrolyte may be a solid electrolyte, or a non-aqueous electrolytic solution and a solid electrolyte may be used in combination.

[0084] The solid electrolyte can be selected from any material that has ionic conductivity of lithium, sodium, calcium, etc. and is solid at room temperature (e.g., 15° C. to 25° C.) Examples of the solid electrolyte include sulfide solid electrolytes, oxide solid electrolytes, oxynitride solid electrolytes, and polymer solid electrolytes.

[0085] As the sulfide solid electrolyte, for example, Li2S-P2S5, LiI-Li2S-P2S5, Li 10 Ge-P2S 12 etc.

[0086] The shape of the nonaqueous electrolyte storage element of this embodiment is not particularly limited, and examples thereof include cylindrical batteries, prismatic batteries, flat batteries, coin batteries, and button batteries.

[0087] FIG. 6 shows an energy storage element 101 as an example of a prismatic battery. Note that this figure is a see-through view of the inside of the container. An electrode assembly 102 having a positive electrode and a negative electrode wound with a separator sandwiched between them is housed in a prismatic container 103. The positive electrode is electrically connected to a positive electrode terminal 104 via a positive electrode lead 141. The negative electrode is electrically connected to a negative electrode terminal 105 via a negative electrode lead 151.

[0088] In addition, in the energy storage element 101, a tip portion of the reference electrode 106 is inserted between the positive electrode and the negative electrode that constitute the electrode body 102. In one embodiment, the tip portion (the portion including the porous portion) of the reference electrode 106 is disposed between the positive electrode and the negative electrode. The reference electrode 106 can be disposed between the positive electrode and the negative electrode, for example, via a separator, so as not to come into contact with the positive electrode and the negative electrode. The reference electrode 106 is also electrically connected to a reference electrode terminal 107 provided on the upper surface of the container 103. A specific configuration of the reference electrode 106 can be, for example, the same as that of the reference electrode 14 in FIG. 2.

[0089] Since the energy storage element 101 includes the reference electrode 106 having a porous portion with a porosity of 50% or more, the negative electrode potential of the energy storage element 101 can be measured with high accuracy using a conventional device. Similarly, the positive electrode potential of the energy storage element 101 can be measured with high accuracy.

[0090] <Configuration of the power storage device> The energy storage element of this embodiment can be mounted as an energy storage unit (battery module) configured by assembling a plurality of energy storage elements in a power source for an automobile such as an EV, HEV, or PHEV, a power source for electronic devices such as a personal computer or a communication terminal, an industrial power source, a stationary power source, a power source for power storage, etc. In this case, the technology of the present invention may be applied to at least one energy storage element included in the energy storage unit.

[0091] 7 shows an example of a power storage device 130 in which power storage units 120, each of which is an assembly of two or more electrically connected power storage elements 101, are further assembled. The power storage device 130 may include a bus bar (not shown) that electrically connects the two or more power storage elements 101, a bus bar (not shown) that electrically connects the two or more power storage units 120, and the like. The power storage unit 120 or the power storage device 130 may include a state monitoring device (not shown) that monitors the state of one or more power storage elements. An example of the state monitoring device is a device that monitors the negative electrode potential relative to a reference electrode.

[0092] <Method of manufacturing an energy storage element> The method for manufacturing the energy storage element of this embodiment can be appropriately selected from known methods. The manufacturing method includes, for example, preparing an electrode assembly, preparing an electrolyte, and housing the electrode assembly and the electrolyte in a container. Preparing the electrode assembly includes preparing a positive electrode, a negative electrode, and a reference electrode, and forming the electrode assembly by stacking or winding the positive electrode and the negative electrode with a separator interposed therebetween.

[0093] In forming the electrode body, the electrode body can be formed with a reference electrode disposed between the positive electrode and the negative electrode. Alternatively, an energy storage element may be assembled using a reference electrode that does not have a porous portion, and a charging operation may be performed between the positive electrode and the reference electrode as described above, thereby forming a porous portion in the reference electrode.

[0094] <Other embodiments> The charging condition determination method and the energy storage device of the present invention are not limited to the above-described embodiments, and various modifications may be made without departing from the spirit of the present invention. For example, the configuration of one embodiment can be added to the configuration of another embodiment, or part of the configuration of one embodiment can be replaced with the configuration of another embodiment or well-known technology. Furthermore, part of the configuration of one embodiment can be deleted. Also, well-known technology can be added to the configuration of one embodiment.

[0095] In the above embodiment, the case where the energy storage element is used as a chargeable and dischargeable non-aqueous electrolyte secondary battery (for example, a lithium ion secondary battery) has been described, but the energy storage element may be of any type, shape, size, capacity, etc. The present invention can also be applied to various secondary batteries, electric double layer capacitors, lithium ion capacitors, and other capacitors.

[0096] In the above embodiment, the electrode assembly in which the positive electrode and the negative electrode are stacked with a separator interposed therebetween has been described, but the electrode assembly may not include a separator. For example, the positive electrode and the negative electrode may be in direct contact with each other in a state in which a non-conductive layer is formed on the active material layer of the positive electrode or the negative electrode.

[0097] In the charging condition determination method and energy storage device of the above embodiment, the reference electrode is disposed between the positive electrode and the negative electrode, but the reference electrode may be disposed at any location. The energy storage device of the present invention also includes an energy storage device (three-electrode cell) used in the charging condition determination method, such as the three-electrode cell 11 in FIG. 1. The reference electrode used in the charging condition determination method and energy storage device of the present invention may be one made of a material other than a copper wire that is coated with an insulating layer except for the tip, and that has a porous portion provided at the tip. [Industrial Applicability]

[0098] The present invention can be applied to a method for determining charging conditions for an electric storage element used as a power source for an automobile or the like. [Explanation of symbols]

[0099] 10 Experimental equipment 11 Three-electrode cell 12 Positive electrode 13 Negative electrode 14 Reference pole 15 Electrolytes 16 Charge / discharge device 17 Control PC 18 copper wire 18a Tip 19 Insulating layer 20 Porous area 101 Energy storage element 102 Electrode body 103 Container 104 Positive terminal 141 Positive lead 105 Negative terminal 106 Reference pole 107 Reference pole terminal 151 Negative lead 120 Energy Storage Unit 130 Electricity storage device

Claims

1. Providing a three-electrode cell including a positive electrode, a negative electrode, and a reference electrode; Charging the three-electrode cell by controlling the negative electrode potential based on the reference electrode; and determining charging conditions using a profile obtained by the charging operation; Equipped with the reference electrode has a porous portion, The reference electrode has a wire made of a metal that does not alloy with lithium and is covered with an insulating layer except for its tip, and the porous portion is arranged to cover the tip of the wire made of the metal that does not alloy with lithium.

2. A positive electrode, a negative electrode, and a reference electrode are provided. the reference electrode has a porous portion, The reference electrode has a wire made of a metal that does not alloy with lithium and is covered with an insulating layer except for its tip, and the porous portion is arranged to cover the tip of the wire made of the metal that does not alloy with lithium.

Images