JP7757637B2 - Charging condition determination method and storage element - Google Patents

Description

The present invention relates to a charging condition determination method and a storage element.

Non-aqueous electrolyte secondary batteries, typified by lithium-ion secondary batteries, are widely used in electronic devices such as personal computers and communication terminals, as well as automobiles, due to their high energy density. Non-aqueous electrolyte secondary batteries generally have a pair of electrodes electrically isolated by a separator and a non-aqueous electrolyte sandwiched 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.

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 precipitate on the surface of the negative electrode. This metallic lithium precipitation is undesirable as it can cause internal short circuits, etc. Patent Document 1 proposes a method for determining whether metallic lithium has precipitated on the negative electrode surface of such lithium-ion secondary batteries. This method involves detecting the amount of change per unit time in battery voltage during constant current charging and determining whether metallic lithium has precipitated based on this amount of change.

JP 2013-89363 A

On the other hand, in order to suppress the deposition of metallic lithium that occurs during the use of a storage element, it is necessary to set appropriate charging conditions for that storage element at the design stage. Because metallic lithium deposition is particularly likely to occur when rapid charging or low-temperature charging is performed, it is hoped that a method can be developed to determine 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.

The present invention was made based on the above circumstances, and its purpose is to provide a method for determining the conditions under which charging can be performed while suppressing the deposition of metallic lithium, and an energy storage element that can measure the negative electrode potential with high accuracy.

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

Another aspect of the present invention relates to an energy storage element comprising a positive electrode, a negative electrode, and a reference electrode, the reference electrode having a porous portion with a porosity of 50% or more.

One aspect of the present invention provides a method for determining conditions under which charging can be performed while suppressing the deposition of metallic lithium, and an energy storage device that can accurately measure the negative electrode potential.

FIG. 1 is a schematic diagram showing an experimental device used in one embodiment of the charging condition determination method. FIG. 2 is a schematic cross-sectional view showing a reference electrode used in one embodiment of the method for determining charging conditions. 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. 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. FIG. 5 is a graph showing an example of a profile (progression of negative electrode potential and current relative to a 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. FIG. 6 is a perspective view showing an embodiment of an energy storage element. FIG. 7 is a schematic diagram showing an embodiment of an electricity storage device configured by assembling a plurality of electricity storage elements.

First, we will provide an overview of the charging condition determination method and energy storage element disclosed in this specification.

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

This charging condition determination method can determine conditions under which charging can be performed while suppressing the deposition of metallic lithium for the following reasons. Metallic lithium deposition on the negative electrode thermodynamically occurs when the negative electrode potential is 0 V (vs. Li/Li ⁺ ) or less. Therefore, to suppress the deposition of metallic lithium, 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 migration 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 to the surface facing the positive electrode) so as not to inhibit the migration of lithium ions, an accurate measurement of the negative electrode potential cannot be performed. In contrast, this charging condition determination method uses a reference electrode having a porous portion, so that the migration of lithium ions is less inhibited even when the reference electrode is placed between the positive and negative electrodes. Therefore, when accurately determining the negative electrode potential, it is only necessary to consider the IR drop (voltage drop due to the resistance of the electrolyte, etc.) between the negative electrode and the reference electrode, and the negative electrode potential can be measured with high accuracy. Therefore, by using such a reference electrode and determining the charging conditions using a profile obtained by controlling the negative electrode potential 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.

The term "porous" means that the porosity is greater than 10%. That is, a porous portion is a portion having a porosity of greater than 10%. 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, etc., 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 then dried under reduced pressure at room temperature.

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. Using such a reference electrode allows for more accurate measurement of the negative electrode potential, and the reference electrode is also relatively easy to manufacture. Examples of the wire made of a metal that does not alloy with lithium that can be used include copper wire, SUS wire, and cobalt wire. Among these, copper wire is preferred from a cost perspective.

The above charging operation preferably includes constant-current, constant-potential charging. By using the 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.

The porosity of the porous region is preferably 50% or more. Using a reference electrode with a porous region of such high porosity allows for more accurate measurement of the negative electrode potential, and as a result, allows for more accurate determination of charging conditions that are less likely to cause deposition of metallic lithium.

Another aspect of the present invention relates to an energy storage element comprising a positive electrode, a negative electrode, and a reference electrode, the reference electrode having a porous portion with a porosity of 50% or more.

This energy storage element is equipped with a reference electrode having a porous portion with a porosity of 50% or more, allowing for highly accurate measurement of the negative electrode potential. Furthermore, this energy storage element also allows for highly accurate measurement of the positive electrode potential.

A charging condition determination method, energy storage element, energy storage device, and energy storage element manufacturing method according to one embodiment of the present invention, as well as other embodiments, are described in detail below. Note that the names of the components (elementary components) used in each embodiment may differ from the names of the components (elementary components) used in the background art.

<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.

Figure 1 shows an experimental device 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.

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 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 for at least one of the positive electrode 12 and the negative electrode 13. Specific examples of the positive and negative electrodes include the positive and negative electrodes provided in the energy storage element according to one embodiment of the present invention, which will be described later. 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, which will be described later. 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.

In the three-electrode cell 11 of Figure 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. Figure 2 shows a suitable form of the reference electrode 14.

The reference electrode 14 in Figure 2 has a copper wire 18 that is covered with an insulating layer 19 except for the tip 18a. In the reference electrode 14 in Figure 2, the tip 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 to cover the tip 18a (end face) of the copper wire 18.

The material forming the insulating layer 19 is not particularly limited as long as it has insulating properties, and resin, for example, can be used. Insulating refers to not having electrical conductivity. Whether or not a material has "electrical conductivity" is determined by using a volume resistivity of 10 ^Ω ·cm measured in accordance with JIS-H-0505 (1975) as a threshold value. The diameter of the copper wire 18 coated with the insulating layer 19 (including the thickness of the insulating layer 19) is, for example, approximately 10 μm or more and 100 μm or less. Furthermore, using the copper wire 18 for the reference electrode 14, i.e., using metallic copper as the core material of the reference electrode 14, has the advantage that when the porous portion 20 is metallic lithium, the metallic lithium and the metallic copper do not alloy, thereby enabling more accurate acquisition of the potential relative to metallic lithium.

The porous region 20 is the portion that serves as the reference for the potential of the reference electrode 14. 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 a lithium alloy containing other elements.

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 coated with the insulating layer 19, or it can be greater than that. The porous region 20 can be in a flat plate shape. For example, the flat porous region 20 can be approximately circular or rectangular in shape in a plan view, with the tip 18a of the copper wire 18 at its center. The width of the porous region 20 in a plan view (the maximum length in a direction perpendicular to the length L in a plan view) can be, for example, 500 μm or more and 10 mm or less.

When the porous region 20 is metallic lithium, it 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 precipitated as metallic lithium at the tip 18a of the copper wire 18, forming a porous region 20 made of metallic lithium. Furthermore, 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, via a separator, and then performing the charging operation described above while compressing this electrode assembly, thereby forming a flat-plate-shaped porous region 20 made of metallic lithium.

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 perspective 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. 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 reference electrode, etc. during the above-mentioned charging operation.

Regarding the influence of the porosity of the porous region 20, Figures 3 and 4 show simulation results of the evolution of the negative electrode potential relative to the reference electrode during constant-current charging at 10 C or 20 C. Each simulation shows results assuming the porosity of the metallic lithium serving as the porous region in the reference electrode 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 for a charging current of 10 C, and Figure 4 shows the results for a charging current of 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 theoretical 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 reference electrode with 100% porosity is the IR drop value 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 reference electrode with 50% porosity is nearly identical to the negative electrode potential measured relative to a reference electrode with 100% porosity. This allows for highly accurate measurement of the negative electrode potential, and therefore, correcting for the IR drop alone allows for highly accurate determination of the true negative electrode potential. On the other hand, the negative electrode potential measured relative to reference electrodes with 1% and 10% porosity differs significantly from the negative electrode potential measured relative to a reference electrode with 100% porosity, particularly in Figure 4 where the current is large. This indicates that the negative electrode potential cannot be measured accurately, and therefore, correcting for the IR drop alone does not allow for highly accurate determination of the true negative electrode potential.

Next, the charging operation for obtaining a profile (data on the transition of current, etc. during charging) will be described. 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 FIG. 5. In this example, charging is performed at a constant current until the negative electrode potential reaches 0 V (vs. Li/Li ⁺ ), and after the negative electrode potential reaches 0 V (vs. Li/Li ⁺ ), charging is controlled so that this negative electrode potential is maintained.

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 FIG. 5. Specifically, by setting the charging conditions as follows: current A when the amount of charge is between 0 and a, 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, it is possible to obtain charging conditions in which the negative electrode potential is controlled not to become less than 0 V (vs. Li/Li ⁺ ), i.e., the deposition of metallic lithium is suppressed.

For example, when constant-current constant-potential charging or the like is performed, the negative electrode potential during constant-potential charging may be controlled at a potential other than 0 V (vs. Li/Li ⁺ ). For example, the overpotential generated varies depending on the current density, etc., and the potential at which metallic lithium precipitates varies. Therefore, the negative electrode potential during constant-potential charging may be controlled to a predetermined value taking these factors into consideration. Furthermore, the charging conditions may be determined by taking into account a correction for the IR drop in the obtained profile.

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

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

The above-described rapid charging and low-temperature charging can be employed in, for example, automotive power sources (energy storage elements) for electric vehicles (EVs), plug-in hybrid electric vehicles (PHEVs), hybrid electric vehicles (HEVs), and the like, industrial power sources (energy storage elements), and stationary power sources (energy storage elements). 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 (energy storage elements), industrial power sources (energy storage elements), stationary power sources (energy storage elements), and the like.

<Electricity storage element>
An energy storage element according to one embodiment of the present invention includes 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 multiple positive electrodes and multiple negative electrodes are stacked with separators interposed therebetween, or a wound type in which the positive electrode and negative electrode are stacked with separators interposed therebetween and wound. The electrolyte is present in a state impregnated in the positive electrode, negative electrode, and separator. The energy storage element also includes a reference electrode. As an example of an energy storage element, a nonaqueous electrolyte secondary battery (hereinafter simply referred to as a "secondary battery") will be described.

(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.

The positive electrode substrate is conductive. Examples of materials for the positive electrode substrate include metals such as aluminum, titanium, tantalum, and stainless steel, as well as alloys of these. Among these, aluminum or aluminum alloys are preferred from the perspectives of potential resistance, high conductivity, and cost. Positive electrode substrates include foils, vapor-deposited films, meshes, and porous materials, with foils being preferred from the perspective 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).

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

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 composition of the intermediate layer is not particularly limited, and may include, for example, a binder and a conductive agent.

The positive electrode active material layer contains a positive electrode active material. The positive electrode active material layer may also contain optional components such as a conductive agent, binder, thickener, and filler, as needed.

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 typically used as the positive electrode active material for lithium ion secondary batteries. Examples of the positive electrode active material include lithium transition metal composite oxides having an α-NaFeO ₂ -type crystal structure, lithium transition metal composite oxides having a spinel-type crystal structure, polyanion compounds, chalcogen compounds, and sulfur. Examples of lithium transition metal composite oxides having α-NaFeO type ₂ crystal structure include Li[Li _x Ni _(1-x) ]O ₂ (0≦x<0.5), Li[Li _x Ni _γ Co _(1-x-γ) ]O ₂ (0≦x<0.5, 0<γ<1, 0<1-x-γ), Li[Li _x Co _(1-x) ]O ₂ (0≦x<0.5), Li[Li _x Ni _γ Mn _(1-x-γ) ]O ₂ (0≦x<0.5, 0<γ<1, 0<1-x-γ), Li[Li _x Ni _γ Mn _β Co _(1-x-γ-β) ]O ₂ (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) ] O ₂ (0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1, 0<1-x-γ-β), etc. Examples of lithium transition metal composite oxides having a spinel crystal structure include Li _x Mn ₂ O ₄ and Li _x Ni _γ Mn _(2-γ) O ₄ . 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.

The positive electrode active material is typically 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. Setting the average particle size of the positive electrode active material above the lower limit facilitates the production and handling of the positive electrode active material. Setting the average particle size of the positive electrode active material below the upper limit improves the electronic conductivity of the positive electrode active material layer. 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. "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).

To obtain powders of a specified particle size, mills and classifiers are used. Examples of milling methods include those using a mortar, ball mill, sand mill, vibrating ball mill, planetary ball mill, jet mill, counter jet mill, swirling air jet mill, or sieves. Wet milling, in which water or an organic solvent such as hexane is present, can also be used. Sieves and air classifiers are used as classification methods, both dry and wet, as needed.

The content of the positive electrode active material in the positive electrode active material layer is preferably 50% by mass or more and 99% by mass or less, more preferably 70% by mass or more and 98% by mass or less, and even more preferably 80% by mass or more and 95% by mass or less. By keeping 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.

The conductive agent is not particularly limited as long as it is a material that is conductive. Examples of such conductive agents include carbonaceous materials, metals, and conductive ceramics. Carbonaceous materials include graphite, non-graphitic carbon, and graphene-based carbon. Non-graphitic carbon includes carbon nanofiber, pitch-based carbon fiber, and carbon black. Carbon black includes furnace black, acetylene black, and ketjen black. Graphene-based carbon includes graphene, carbon nanotubes (CNT), and fullerene. 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 CNT may be used. Among these, carbon black is preferred from the standpoints of electronic conductivity and coatability, and acetylene black is particularly preferred.

The content of the conductive agent in the positive electrode active material layer is preferably 1% by mass or more and 10% by mass or less, and more preferably 3% by mass or more and 9% by mass or less. By keeping the content of the conductive agent within this range, the energy density of the energy storage element can be increased.

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.

The binder content in the positive electrode active material layer is preferably 1% by mass or more and 10% by mass or less, and more preferably 3% by mass or more and 9% by mass or less. By keeping the binder content within this range, the active material can be stably maintained.

Examples of thickeners include polysaccharide polymers such as carboxymethyl cellulose (CMC) and methyl cellulose. If 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.

The filler is not particularly limited. Examples of fillers 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; mineral-derived substances such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica; and synthetic versions of these.

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.

(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.

The negative electrode substrate is electrically conductive. Materials used for the negative electrode substrate include metals such as copper, nickel, stainless steel, nickel-plated steel, and aluminum, or alloys of these metals, and carbonaceous materials. Among these, copper or copper alloys are preferred. Examples of negative electrode substrates include foil, vapor-deposited film, mesh, and porous materials, with foil being preferred from a cost perspective. 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.

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

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

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.

The negative electrode active material can be appropriately selected from known negative electrode active materials. A material capable of absorbing and releasing lithium ions is typically used as the negative electrode active material 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; titanium-containing oxides such as Li ₄ Ti ₅ O ₁₂ , LiTiO _{2 , and} TiNb ₂ O ₇ ; polyphosphate compounds; silicon carbide; 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 mixed and used.

"Graphite" refers to a carbon material in which the average lattice spacing (d ₀₀₂ ) of the (002) plane, as determined by X-ray diffraction before charge/discharge or in a discharged state, 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.

"Non-graphitic carbon" refers to a carbon material in which the average lattice spacing (d ₀₀₂ ) of the (002) plane, as determined by X-ray diffraction before charge/discharge or in a discharged state, is 0.34 nm or more and 0.42 nm or less. Examples of non-graphitic carbon include 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.

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

The term "non-graphitizable carbon" refers to a carbon material having the above d ₀₀₂ of 0.36 nm or more and 0.42 nm or less.

The term "easily graphitizable carbon" refers to a carbon material having the above d ₀₀₂ of 0.34 nm or more and less than 0.36 nm.

The negative electrode active material is typically in the form of particles (powder). The average particle size of the negative electrode active material can be, for example, 1 nm to 100 μm. 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 to 100 μm. When the negative electrode active material is Si, Sn, Si oxide, Sn oxide, or the like, the average particle size may be 1 nm to 1 μm. Setting the average particle size of the negative electrode active material to be above the lower limit facilitates the production and handling of the negative electrode active material. Setting the average particle size of the negative electrode active material to be below the upper limit improves the electronic conductivity of the active material layer. To obtain powder with a specified particle size, a pulverizer, a classifier, or the like is used. The pulverization and classification methods can be selected from, for example, the methods exemplified for the positive electrode above. 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.

The content of the negative electrode active material in the negative electrode active material layer is preferably 60% by mass or more and 99% by mass or less, and more preferably 90% by mass or more and 98% by mass or less. By keeping the content of the negative electrode active material within this range, it is possible to achieve both high energy density and manufacturability of the negative electrode active material layer.

(Reference pole)
The reference electrode has a porous portion with a porosity of 50% or more. Specific and preferred embodiments of the reference electrode provided in the energy storage element include the reference electrode described as the reference electrode used in the charging condition determination method according to one embodiment of the present invention, in which the porosity of the porous portion is 50% or more.

(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. As the material for the substrate layer of the separator, polyolefins such as polyethylene and polypropylene are preferred from the viewpoint of shutdown function, and polyimide and aramid are preferred from the viewpoint of oxidative decomposition resistance. A composite material of these resins may also be used as the substrate layer of the separator.

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 less than the specified value 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 aluminosilicates; 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 may be mixed and used. Of these inorganic compounds, silicon oxide, aluminum oxide, or aluminosilicate are preferred from the perspective of the safety of the energy storage element.

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

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

(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.

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 in these compounds have been substituted with halogens may also be used.

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, and 1,2-diphenylvinylene carbonate. Of these, EC is preferred.

Examples of chain carbonates include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diphenyl carbonate, trifluoroethyl methyl carbonate, and bis(trifluoroethyl) carbonate. Of these, EMC is preferred.

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 promotes dissociation of the electrolyte salt, improving the ionic conductivity of the non-aqueous electrolyte. The use of a chain carbonate reduces 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.

The electrolyte salt can be appropriately selected from known electrolyte salts. Examples of electrolyte salts include lithium salts, sodium salts, potassium salts, magnesium salts, and onium salts. Of these, lithium salts are preferred.

Examples of the lithium salt include inorganic lithium salts such as _LiPF6 , _{LiPO2F2 , LiBF4} , _LiClO4 , and _LiN ( _SO2F ) ₂ ; lithium oxalate salts 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.

The content of the electrolyte salt in the non-aqueous electrolyte solution at 20°C and 1 atmosphere is preferably 0.1 mol/dm3 or ^more and 2.5 mol/dm3 ^{or less} , more preferably 0.3 mol/dm3 or ^more and 2.0 mol/dm3 ^{or less} , even more preferably 0.5 mol/dm3 or ^more and 1.7 mol/dm3 or ^less , and particularly preferably 0.7 mol/dm3 or ^more and 1.5 mol/ ^dm3 or less. By setting the content of the electrolyte salt within the above range, the ionic conductivity of the non-aqueous electrolyte solution can be increased.

In addition to the nonaqueous solvent and electrolyte salt, the nonaqueous electrolyte may contain additives. Examples of additives include halogenated carbonates such as fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC); oxalates such as lithium bis(oxalate)borate (LiBOB), lithium difluorooxalateborate (LiFOB), and lithium bis(oxalate)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, methylvinylene carbonate, ethylvinylene 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- Examples of additives include tetrakis(trimethylsilyl)silyl ether, tetrakis(trimethylsilyl) ...

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

A solid electrolyte may be used as the non-aqueous electrolyte, or a non-aqueous electrolyte solution and a solid electrolyte may be used in combination.

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

In the case of a lithium ion secondary battery, examples of sulfide solid electrolytes include Li ₂ SP ₂ S ₅ , LiI-Li ₂ SP ₂ S ₅ , and Li ₁₀ Ge-P ₂ S ₁₂ .

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

Figure 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.

In addition, in the energy storage element 101, the tip portion of the reference electrode 106 is inserted between the positive and negative electrodes that make up the electrode body 102. In one embodiment, the tip portion of the reference electrode 106 (the portion including the porous portion) is disposed between the positive and negative electrodes. The reference electrode 106 can be disposed between the positive and negative electrodes, for example, via a separator, so as not to come into contact with the positive and negative electrodes. The reference electrode 106 is also electrically connected to a reference electrode terminal 107 provided on the top 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 Figure 2.

The energy storage element 101 includes a reference electrode 106 having a porous portion with a porosity of 50% or more, so the negative electrode potential of the energy storage element 101 can be measured with high accuracy using conventional equipment. Similarly, the positive electrode potential of the energy storage element 101 can also be measured with high accuracy.

<Configuration of 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 an automobile power source 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, or 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.

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

<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.

When forming the electrode body, the electrode body can be formed with a reference electrode placed between the positive electrode and negative electrode. Alternatively, a storage element can be assembled using a reference electrode that does not have a porous portion, and a porous portion can be formed in the reference electrode by performing a charging operation between the positive electrode and the reference electrode as described above.

<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.

In the above embodiment, the energy storage element is described as being used as a chargeable and dischargeable non-aqueous electrolyte secondary battery (e.g., a lithium-ion secondary battery), but the type, shape, dimensions, capacity, etc. of the energy storage element are arbitrary. The present invention can also be applied to various secondary batteries, electric double-layer capacitors, lithium-ion capacitors, and other capacitors.

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

In addition, in the charging condition determination method and energy storage element of the above embodiment, the reference electrode is placed between the positive electrode and the negative electrode, but the reference electrode may be placed anywhere. The energy storage element of the present invention also includes an energy storage element (three-electrode cell) used in the charging condition determination method, such as the three-electrode cell 11 in Figure 1. Furthermore, the reference electrode used in the charging condition determination method and energy storage element of the present invention may be made of a material other than copper wire that is coated with an insulating layer except for the tip, and that has a porous portion at the tip.

The present invention can be applied to methods for determining charging conditions for storage elements used as power sources for automobiles, etc.

10 Experimental apparatus 11 Three-electrode cell 12 Positive electrode 13 Negative electrode 14 Reference electrode 15 Electrolyte 16 Charging/discharging device 17 Control PC
18 Copper wire 18a Tip portion 19 Insulating layer 20 Porous portion 101 Energy storage element 102 Electrode body 103 Container 104 Positive electrode terminal 141 Positive electrode lead 105 Negative electrode terminal 106 Reference electrode 107 Reference electrode terminal 151 Negative electrode lead 120 Energy storage unit 130 Energy storage device

Claims (2)

Providing 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,
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. 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.