Patent Application
20240274807
The present disclosure relates to a wound non-aqueous electrolyte battery.
As a structure of a negative electrode of a non-aqueous electrolyte battery, a negative electrode including an aluminum foil attached to lithium and a negative electrode including lithium-aluminum alloy (Li—Al alloy) have been proposed.
PTL 1 describes that, in manufacturing the non-aqueous electrolyte battery including lithium-aluminum alloy for the negative electrode, a non-aqueous electrolyte battery with good productivity and good load characteristics until an end stage of a discharging period by cutting a foil of lithium-aluminum alloy containing lithium of 80 atom % or more into a rectangular shape and placing the rectangular lithium-aluminum alloy piece in a negative electrode can to perform rolling.
PTL 2 discloses a negative electrode including lithium as active material that is used for an organic electrolyte primary battery and aluminum placed on a positive electrode side of a surface of lithium. Lithium is defused toward aluminum and Li—Al alloy is formed electrochemically. PTL 2 employs aluminum with temper designation of H16 or H28 in accordance with JIS H 0001, which is entirely or mostly without annealing treatment after rolling. This configuration provides a battery with low internal resistance and good pulse discharge characteristics.
PTL 3 proposes the use of light metal powder with an average grain size ranging from 10 μm to 300 μm or a porous aggregate of light metal alloy powder (e.g., Li—Al alloy particles) for the negative electrode in order to improve heavy load characteristics of the non-aqueous electrolyte battery.
In a method disclosed in PTL 1, a Li—Al alloy negative electrode is placed in a negative can and then rolled to press-bond the Li—Al alloy negative electrode inside the negative can. However, this method is hardly applied to a battery employing a wound electrode group in which a negative plate, a positive plate, and a separator are wound. In addition, a grain boundary of the Li—Al alloy tends to crack at rolling the negative electrode onto the negative can. This may decrease discharge characteristics.
In a battery disclosed in PTL 2, the Li—Al alloy is pulverized and a gap exists between interfaces of Li—Al alloy particles. In a deep discharge state (e.g., depth of charge (DOD) of 90% or more), the gap expands and an electrical connection path between the particles tends to break. As a result, similar to PTL 1, conductivity of the negative electrode at an end stage of a discharging period decreases, and output characteristics at low temperatures may decrease. In a battery disclosed in PTL 3, a gap exists between the Li—Al alloy particles, similar to the battery of PTL2, and thus the output characteristics at low temperatures may decrease at an end stage of the discharging period.
A wound non-aqueous electrolyte battery in one aspect of the present disclosure includes a positive electrode, a negative electrode, a separator provided between the positive and negative electrodes, and a non-aqueous electrolyte. The negative electrode includes a lithium-aluminum alloy sheet. The lithium-aluminum alloy sheet has surfaces including a surface of which parts are flush and connected with one another via grain boundaries of lithium-aluminum alloy. An average grain boundary number N is 8.75 or less, the average grain boundary number N being provided as ΣGi/ΣLi by dividing ΣGi by ΣLi, where ΣGi is a sum of numbers of grain boundaries Gi out of the plurality of grain boundaries which intersect M line segments PQi respectively connecting point Pi to point Qi on the surface of the lithium-aluminum alloy sheet, ΣLi is a sum of lengths Li of the M line segments PQi, M is an integer greater than 1, and i is an integer satisfying 1<i≤M).
The present disclosure a non-aqueous electrolyte battery maintaining high output characteristics thereof at low temperatures also at an end stage of the discharging period.
A wound non-aqueous electrolyte battery (hereinafter also simply referred to as a “non-aqueous electrolyte battery) in accordance with an exemplary embodiment of the present disclosure includes a positive electrode, a negative electrode, a separator provided between the positive electrode and the negative electrode, and a non-aqueous electrolyte. The negative electrode includes a lithium-aluminum alloy sheet (a Li—Al alloy sheet). The wound non-aqueous electrolyte battery often includes a wound electrode group including strips of the positive electrode and the negative electrode spirally wound with the separator therebetween.
Surfaces of the Li—Al alloy sheet includes a surface of which parts are flush and connected with one another via grain boundaries of lithium-aluminum alloy. This configuration maintains electrical connection between Li—Al alloy particles even at an end stage of a discharging period.
In a microscopic view, a surface of the Li—Al alloy sheet includes exposed surfaces of the Li—Al alloy particles separated via grain boundaries, and the exposed surfaces are connected to one another. Normally, the exposed surfaces of the Li—Al alloy particles are not parallel to one another, and therefore, provide the surface of the Li—Al alloy sheet with fine roughness. In addition, a porous Li—Al alloy sheet has a gap between the Li—Al alloy particles.
Surface of the Li—Al alloy sheet including a surface of which parts are flush and connected with one another via grain boundaries implies, firstly, that the Li—Al alloy sheet is not porous and has substantially no gap between the Li—Al alloy particles. In this case, the exposed surfaces of the Li—Al alloy particles are mutually connected via the grain boundaries to form a single surface of the Li—Al alloy sheet as a whole. Secondly, the exposed surfaces of the Li—Al alloy particles are substantially parallel to one another and are also flush and connected with one another substantially on the same plane. This means that the surface of the Li—Al alloy sheet is flat substantially without roughness. A surface roughness of the Li—Al alloy sheet may be, for example, not greater than 0.8 μm or not greater than 0.4 μm in arithmetic average roughness Ra specified by JIS B 0061:2001.
During discharge, the lithium tends to be preferentially consumed from parts thereof close to the grain boundaries of the Li—Al alloy sheet. In the case that the surface of the Li—Al alloy sheet has fine dents or gaps therein caused by the grain boundaries, lithium in the dents tends to be preferentially consumed, and a depth of each dent increases or a gap is expanded. As a result, electrical connection between the Li—Al alloy particles may be cut. In contrast, the surface of the Li—Al alloy sheet of which parts are flush and connected with one another surface via grain boundaries allows the battery to be discharged evenly such that the thickness of the Li—Al alloy sheet evenly decreases over the entire surface area during discharge. As a result, a thin film of the Li—Al alloy sheet remains even at an end stage of the discharging period, and maintains the electrical connection between the Li—Al alloy particles.
Two arbitrary points P and Q on the surface of the Li—Al alloy sheet are selected. Length L (mm) is a length of straight line segment PQ. Points P and Q are preferably selected to allow the line segment to have length L of 1 mm or more. A metal structure of the surface of the Li—Al alloy sheet is observed with a microscope or the like to identify grain boundaries by image processing of a captured image. Then, the number G of grain boundaries of lithium-aluminum alloy intersecting line segment PQ is obtained. More specifically, M groups of points Pi and Qi (Points P1 and Q1, points P2 and Q2, . . . , points PM and QM) are selected where M is an integer greater than 1 and i is an integer satisfying 1<i≤M. The number Gi (G1, G2, . . . , GM) of grain boundaries intersecting line segments PQi connecting point Pi to Qi and lengths Li (L1, L2, . . . , LM) (mm) of line segments PQi (PQ1, PQ2, . . . , PQM) are obtained. The points may be preferably selected such that at least two groups out of plural groups of points Pi and Qi form line segments PQi not parallel to one another. In other words, points Pi and Qi may be preferably selected such that at least two line segments out of M line segments PQi are not parallel to one another. With respect to M line segments PQi, the sum ΣGi (ΣGi=G1+G2+ . . . +GM) of the numbers Gi (G1, G2, . . . , GM) of the grain boundaries is divided by the sum ΣLi(ΣLi=L1+L2+ . . . +LM) of length Li (L1, L2, . . . , LM) of line segments PQi (PQ1, PQ2, . . . , PQM), which is ΣGi/ΣLi, to obtain an average grain boundary number N. Sum ΣLi of length Li is preferably 3 mm or more. The average grain boundary number N is 8.75 or less. As described above, the average grain boundary number N is 8.75 or less when ΣGi/ΣLi, in the plurality of grain boundaries, is calculated by dividing the sum ΣGi of numbers Gi of grain boundaries intersecting M line segments PQi, respectively connecting points Pi and Qi on the surface of the lithium-aluminum alloy sheet by sum ΣLi of lengths Li of M line segments PQi. Average grain boundary number N is preferably equal to or more than 3.12 and equal to or less than 8.75, more preferably is equal to or more than 3.12 and equal to or less than 7.5, and is further preferably equal to or more than 3.12 and equal to or less than 6.25.
The average grain boundary number N of 8.75 or less sufficiently reduces the sum of grain boundary lengths in the Li—Al alloy sheet, and suppresses an increase of internal resistance in the non-aqueous electrolyte battery due to grain boundaries becoming a resistance component. As a result, high output characteristics are also maintained even at an end stage of a discharging period.
In this example, a rectangular region of 1.28 mm×0.96 mm on the surface of the Li—Al alloy sheet is selected. This rectangular region has two diagonal lines α and β. An average grain boundary N is calculated according to the following formula where number Gα is the number of grain boundaries of lithium-aluminum alloy intersecting one diagonal line α, number Gβ is the number of grain boundaries of lithium-aluminum alloy intersecting another diagonal line β, and L (mm) is a length of diagonal lines α and β.
Since length L of diagonal lines α and β is 1.6 mm, the average grain boundary number N is 8.75 or less if (Gα+Gβ)/2 is 14 or less. Thus, high output characteristics is maintained at an end stage of the discharging period. Accordingly, average grain boundary number N is 8.75 or less when (Gα+Gβ)/2 is 14 or less. (Gα+Gβ)/2 is preferably equal to or more than 5 and equal to or less than 14, and is more preferably equal to or more than 5 and equal to or less than 12 or is equal to or more than 5 and equal to or less than 10.
As difference |Gα−Gβ| between number Gα and number Gβ becomes greater, a difference in grain size of Li—Al alloy particles in a direction of diagonal line α and a direction of diagonal line β increases, which means larger anisotropy. As an indicator for evaluating anisotropy, grain boundary aspect ratio A expressed by A=|Gα−Gβ|/(Gα+Gβ) is obtained. Grain boundary aspect ratio A is preferably 0.2 or less. In this case, an effect of maintaining high output characteristics is enhanced even at an end stage of the discharging period. More specifically, high output characteristics is maintained in a deeper depth of discharge (e.g., depth of discharge of 95% or more).
The Li—Al alloy sheet satisfying the above relation of the average grain boundary number (or relation of Gα and Gβ) and having the surface of which parts are flush and connected with one another via grain boundaries is manufactured, for example, using a Li—Al alloy ingot obtained by cooling a molten liquid of Li and Al. By controlling a cooling speed (i.e., solidification speed) of the molten liquid, average grain boundary number N (or (Gα+Gβ)/2) of the ingot is controlled to a desired value.
The Li—Al alloy ingot is, for example, formed into a sheet with a predetermined thickness by extrusion molding to provide the Li—Al alloy sheet. Extrusion molding provides the surface of which parts are flush and connected with one another via grain boundaries on the surface of the Li—Al alloy sheet. Difference |Gα−Gβ| may be controlled by controlling an extrusion speed of extrusion molding. As a result, grain boundary aspect ratio A is controlled to have a required value.
The surface of which parts are flush and connected with one another via the grain boundaries preferably occupies 80% or more of the surface of the Li—Al alloy sheet. More specifically, when ten arbitrary regions of 1.28 mm×0.96 mm on the surface of Li—Al alloy sheet are selected, eight or more regions out of the ten regions preferably satisfy the above relation of the average grain boundary number N.
An Al content in the Li—Al alloy is preferably equal to or more than 0.3 mass % and equal to or less than 1.0 mass %. Average grain boundary number N (or (Gα+Gβ)/2) may decrease by reducing the Al content, but the grain boundary aspect ratio A tends to increase. On the other hand, larger Al content may easily reduce grain boundary aspect ratio A, but hardly provides average grain boundary number N of 8.75 or less (or (Gα+Gβ)/2 of 14 or less). In the case that the Al content exceeds 1.0 mass %, the cooling speed of molten liquid needs to be made slow more than necessary in order to obtain Li—Al alloy with small average grain boundary number N. This reduces production efficiency. However, I the case that the Al content equal to or more than 0.3 mass % and equal to or less than 1.0 mass %, (Gα+Gβ)/2 and grain boundary aspect ratio A may easily controlled without decreasing production efficiency and the effect of maintaining high output characteristics can be enhanced also in the discharge terminal period.
The Li—Al alloy sheet is preferably not rolled, except for the above-described extrusion molding. Rolling may cause fine cracks in the Li—Al alloy sheet. This may generate gaps between the Li—Al alloy particles, and the gaps expand by discharge. Expanded gaps may cut electrical connection paths between the particles. As a result, the output characteristics at low temperatures may decrease. However, the unrolled Li—Al alloy sheet suppresses a decrease in the output characteristics also in the discharge terminal period and improve the output characteristics at low temperatures.
The present disclosure is applicable to any wound non-aqueous electrolyte battery, regardless of a primary or secondary battery and structures of the positive and negative electrodes, as long as the negative electrode includes the above-described Li—Al alloy sheet. In particular, in the case that the present disclosure is applied to a lithium primary battery including at least one of metal lithium and lithium alloy in the negative electrode, a high capacity battery with good discharge characteristics can be achieved.
The battery of the present disclosure is not particularly limited to the wound non-aqueous electrolyte battery. The non-aqueous electrolyte battery according to the exemplary embodiment will be further detailed as a cylindrical lithium primary battery as an example.
The positive electrode may include a positive mixture layer and a positive collector that holds the positive mixture layer. The positive collector is, for example, an expand metal, a net, or a punching metal of stainless steel. The positive mixture layer is obtained by, for example, adding an appropriate amount of water to a positive active material and additive to prepare a wet positive mixture, pressurizing the wet positive mixture in a thickness direction so as to fill a mesh of expand metal, and then drying and rolling the positive mixture to a predetermined thickness.
The positive active material included in the positive electrode is, for example, manganese dioxide. The positive electrode including manganese dioxide generates a relatively high voltage and has good pulse discharge characteristics. Manganese dioxide may also be in a mixed crystal state containing plural types of crystal states. The positive electrode may contain manganese oxides other than manganese dioxide. Manganese oxides other than manganese dioxide are, for example, MnO, Mn3O4, Mn2O3, and Mn2O7. A main component of manganese oxides included in the positive electrode is preferably manganese dioxide.
Lithium may be doped in part of manganese dioxide included in the positive electrode. A small lithium doping quantity secures high capacity. Manganese dioxide and manganese dioxide doped with a small quantity of lithium is be expressed as LixMnO2 (0≤x≤0.05). An average composition of overall manganese oxides included in the positive electrode may be LixMnO2 (0≤x≤0.05). Proportion x of Li in an initial discharge period of the lithium primary battery may be 0.05 or less. In general, proportion x of Li increases as discharge of the lithium primary battery advances. The oxidation number of manganese contained in manganese dioxide is theoretically 4. However, the oxidation number of manganese may become smaller than 4 due to manganese dioxide being doped with lithium. Therefore, an average oxidation number of manganese in LixMnO2 is acceptable to be slightly less than 4.
The positive electrode main contain other positive active materials used in the lithium primary battery. For example, the other positive active material may be, e.g., graphite fluoride. A percentage of LixMnO2 in the whole positive active material may be 90 mass % or more.
Electrolytic manganese dioxide is preferably used as manganese dioxide. As required, electrolytic manganese dioxide subjected to at least neutralization, cleaning, or firing may be used. In general, electrolytic manganese dioxide is obtained by electrolyzing manganese sulfate aqueous solution.
Crystallinity of manganese dioxide may increase and a specific surface area of electrolytic manganese dioxide may decrease by adjusting electrolytic synthesis condition. ABET specific surface area of LixMnO2 may be equal to or more than 10 m2/g and equal to or less than 50 m2/g. The BET specific surface area of LixMnO2 in this range suppresses a voltage drop during pulse discharge and suppresses gas generation during high-temperature storage in the lithium primary battery. In addition, the positive mixture layer can be easily formed.
The BET specific surface area of LixMnO2 is measured by a known method. For example, a specific surface area measuring apparatus (e.g., by MOUNTECH Co., Ltd.) may be used for measurement according to the BET method. For example, LixMnO2 separated from the positive electrode taken out from the battery can be used as a measuring sample.
A median of particle sizes of LixMnO2 may be equal to or more than 15 μm and equal to or less than 80 μm. The median of particle sizes (median diameter D50) in this range allows LixMnO2 that is the positive active material to be connected to the collector (expand metal) via a large amount of conductive assistant to enhance current collecting performance. Still more, a decrease in current collecting performance due to uneven distribution of the conductive assistant in gaps between the particles caused by reduced mixture density can be suppressed. As a result, the discharge performance is enhanced and a voltage drop during pulse discharge is suppressed.
The median of particle sizes of LixMnO2 is a median of grain size distribution obtained by, for example, the quantitative laser diffraction method (qLD method). For example, LixMnO2 separated from the positive electrode taken out from the battery is used as a measuring sample. For measurement, for example, SALD-7500 nano by SHIMADZU Corporation is used.
The positive mixture may further contain a binder in addition to the positive active material. The positive mixture may contain a conductive agent.
The binder is, for example, fluorine resin, rubber particle, or acryl resin.
The conductive agent is, for example, conductive carbon material. The conductive carbon material is, for example, natural graphite, artificial graphite, carbon black, and carbon fiber.
The negative electrode includes the lithium-aluminum alloy sheet. The surface of the lithium-aluminum alloy sheet includes a surface of which parts are flush and connected to one another via grain boundaries as described above. The negative electrode may further contain, in addition to the lithium-aluminum alloy sheet, metal lithium and/or lithium alloy other than lithium-aluminum alloy. The negative electrode may be a multilayer sheet including the lithium-aluminum alloy sheet and a sheet containing metal lithium and/or lithium alloy other than lithium-aluminum alloy.
The lithium-aluminum alloy sheet is an alloy sheet including Li and Al. However, the sheet may include elements other than Al that can form alloy with lithium. In the lithium-aluminum alloy sheet, the Al content preferably ranges from 0.3 mass % to 1.0 mass %. In the case that the lithium-aluminum alloy sheet contains an element other than Al that forms alloy with lithium, the content of this element is smaller than the Al content and is, for example, 0.2 mass % or less.
The lithium-aluminum alloy may contain a small quantity of element, such as Sn, Ni, Si, or Pb, that forms alloy with lithium.
The negative electrode including the lithium-aluminum alloy sheet is formed into a predetermined shape and thickness according to the shape, dimensions, and specified performance of the lithium primary battery. More specifically, a cylindrical battery uses, for example, a foil of lithium-aluminum alloy sheet that has a longer direction and a shorter direction.
For the cylindrical battery, a long tape including having a resin base material and an adhesive layer may be attached in the longer direction of at least one main surface of the negative electrode. The main surface refers to a surface facing the positive electrode. A width of this tape is, for example, equal to or more than 0.5 mm and equal to or less than 3 mm. This tape prevents occurrence of a current collection failure due to thinned and cut alloy sheet of the negative electrode when the lithium component in the negative electrode is consumed by reaction in the discharge terminal period.
A material of the resin base material is, for example, fluorine resin, polyimide, polyphenylene sulfide, polyether sulfone, polyolefin such as polyethylene and polypropylene, or polyethylene terephthalate. In particular, the use of polyolefin is preferable, and polypropylene is more preferable.
The binder layer contains, for example, at least one type of component selected from the group of rubber component, silicone component, and acryl resin component. More specifically, synthetic rubber or natural rubber may be used as the rubber component. The synthetic rubber is, for example, butyl rubber, butadiene rubber, styrene-butadiene rubber, isoprene rubber, neoprene, polyisobutylene, acrylonitrile-butadiene rubber, styrene-isoprene block copolymer, styrene-butadiene block copolymer, and styrene-ethylene-butadiene block copolymer. An organic compound having polysiloxane structure, silicone polymer, or the like is used as the silicone component. The silicone polymer is, for example, peroxide-curing type silicone and addition-reaction type silicone. Polymer of acryl monomer, such as acrylic acid, methacrylic acid, acrylic acid ester, and methacrylic acid ester, can be used as the acryl resin component. This includes single polymer or copolymer of acryl monomer such as acrylic acid, methacrylic acid, methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, butyl acrylate, butyl methacrylate, octyl acrylate, octyl methacrylate, 2-ethylhexyl acrylate, and 2-ethylhexyl methacrylate. Note that the binder layer may contain crosslinking agent, plasticizer, and tackifier.
The electrolyte (non-aqueous electrolyte) is a non-aqueous electrolytic solution in which lithium salt or lithium ion is dissolved in a non-aqueous solvent.
The non-aqueous solvent is organic solvent that is generally used for the non-aqueous electrolyte of the lithium primary battery. The non-aqueous solvent is, for example, ether, ester, or carbonic ester. Dimethyl ether, γ-butyl lactone, propylene carbonate, ethylene carbonate, or 1, 2-dimethoxyethane can be used as the non-aqueous solvent. The non-aqueous electrolyte may include one type or two or more types of non-aqueous solvent.
In order to enhance the discharge characteristics of the lithium primary battery, the non-aqueous solvent preferably contains cyclic carbonic ester that has high boiling point and chain ether that has low viscosity even at low temperatures. Cyclic carbonic ester preferably includes at least one type selected from the group of propylene carbonate (PC) and ethylene carbonate (EC), and more preferably includes PC. Chain ester preferably has a viscosity of 2 mPa·s or less at 25° C. In particular, chain ester preferably includes dimethoxyethane (DME). A viscosity of the non-aqueous solvent is measured at 25° C. and at a shear rate of 10,000 (1/s), with small sample viscometer m-VROC by RheoSense, Inc.
Lithium salt is, for example, lithium salt that is used as a solute of the lithium primary battery. This lithium salt is, for example, LiCF3SO3, LiN(CF3SO2)2, LiClO4, LiBF4, LiPF6, LiRaSO3 (Ra is fluorinated alkyl group with 1 to 4 carbons), LiFSO3, LiN(SO2Rb)(SO2Rc) (Rb and Rc are fluorinated alkyl group each having 1 to 4 carbons), LiN(FSO2)2, LiPO2F2, LiB(C2O4)2, and LiBF2(C2O4). The non-aqueous electrolyte may include one type or two or more types of these lithium salts.
A concentration of lithium ion in the electrolyte (total concentration of lithium ion) ranges is, for example, from 0.2 mol/L to 2.0 mol/L and may range from 0.3 mol/L to 1.5 mol/L.
The electrolyte may contain an additive as required. The additive is, for example, propanesultone and vinylene carbonate. The total concentration of the additive in the non-aqueous electrolyte ranges, for example, from 0.003 mol/L to 0.5 mol/L.
The lithium primary battery often includes the separator provided between the positive electrode and the negative electrode. The separator may be made of a porous sheet made of an insulating material resistant to inner environment of the lithium primary battery. Specifically, for example, the separator is made of synthetic resin-made nonwoven fabric, synthetic resin-made microporous film, or their laminate.
Synthetic resin used for the nonwoven fabric is, for example, polypropylene, polyphenylene sulfide, and polybutylene terephthalate. Synthetic resin used for the microporous film is, for example, polyethylene, polypropylene, or polyolefin resin such as ethylene-propylene copolymer. The microporous film may include inorganic particles as required.
A thickness of the separator is, for example, equal to or more than 5 μm and equal to or less than 100 μm.
Examples and comparative examples of the present disclosure will be specifically described below. However, the present disclosure is not limited to the following examples.
100 parts by mass of electrolytic manganese dioxide and 5 parts by mass of ketjen black, a conductive agent, were mixed, and then 5 parts by mass of polytetrafluoroethylene, a binder, and an appropriate amount of pure water were added and kneaded, thereby preparing a wet positive mixture.
The wet positive mixture was applied onto the positive collector made of stainless steel expand metal, and then dried to obtain a positive electrode precursor. Then, the positive electrode precursor was rolled by a roll press to obtain a positive electrode with a predetermined positive mixture density. A thickness of the positive electrode after rolling was adjusted to 0.5 mm.
Then, the positive electrode was cut into strips of 200 mm long×38 mm wide. A portion of the positive mixture filled was peeled, and a stainless steel tab lead was resistance-welded to an exposed portion of the positive collector.
The Li—Al alloy sheet was prepared by the following method. The Li—Al alloy sheet was prepared in atmosphere of argon, an inactive gas. First, Li was heated to 350° C. to melt, and then a predetermined proportion of Al was mixed with Li to obtain a molten liquid of Li and A1. The molten liquid of Li and Al was put into a mold, and then, the mold containing the molten liquid was cooled to 150° C. and kept at 150° C. for a predetermined period of time. Then, after cooling the mold to a room temperature, a Li—Al alloy ingot was taken out from the mold. The Al content in the molten liquid was 0.3 mass % with respect to whole Li and Al.
The Li—Al alloy ingot was processed into a sheet with a thickness of 0.2 mm by extrusion molding at an extrusion speed of 50 m/minute to obtain the negative electrode. The surface of the negative electrode obtained is entirely a single surface of which parts connected via grain boundaries. Then, the negative electrode was cut into strips of 210 mm long×width 36 mm wide. A nickel tab lead was connected to a predetermined point of the negative electrode by pressure-welding.
The positive electrode and the negative electrode were stack on each other across the separator and spirally wound to prepare a columnar electrode group with a diameter of 16 mm. The separator was a polyethylene microporous film with a thickness of 20 μm.
PC and DME were mixed to provide a mixture at a volume ratio of 1:2. Lithium trifluoromethanesulfonate (LiCF3SO3) was dissolved in the mixture and adjusted to a concentration of 0.6 mol/L to prepare the non-aqueous electrolyte.
A cylindrical battery case made of nickel-plated steel plate with a bottom was prepared to have a predetermined size. The electrode group was inserted into the battery case with a lower insulation plate with a ring shape disposed at the bottom. Then, the tab lead of the positive electrode was connected to an inner surface of the sealing plate, and the tab lead of the negative electrode was connected to an inner bottom surface of the battery case.
Next, the non-aqueous electrolyte was put in the battery case, and the upper insulation plate was disposed over the electrode group. Then, an opening of the battery case was sealed with the sealing plate. Then, preparatory discharge was performed on each battery to adjust a battery voltage to 3.2 V. The lithium primary batteries (diameter: 17 mm, height: 45 mm) for testing a design capacity of 2500 mAh were thus completed.
In the preparation of the negative electrode, a duration of keeping the molten liquid of Li and Al at 150° C. in the ingot was changed to obtain plural types of Li—Al alloy ingots. These ingots were used for the negative electrode to prepare lithium primary batteries A1-A4, B1, and B2 with different negative electrode structures.
The duration of keeping the molten liquid at 150° C. was 1 hour, 2 hours, 4 hours, and 8 hours in lithium primary batteries A1-A4, respectively. Lithium primary battery B1 was cooled from 350° C. to a room temperature without keeping the molten liquid at 150° C. In lithium primary battery B2, the molten liquid was kept at 150° C. for 30 minutes.
Surfaces of the negative electrodes (Li—Al alloy) prepared were observed with a SEM. Gα and Gβ were calculated by the aforementioned method from a rectangular region of 1.28 mm×0.96 mm to calculate (Gα+Gβ)/2 and grain boundary aspect ratio A (=|Gα−Gβ|/(Gα+Gβ)). The longer the duration of keeping the molten liquid at 150° C., the more a crystal grain of Li—Al alloy grows. As a result, the average grain boundary number N (and (Gα+Gβ)/2) becomes smaller.
As described above, lithium primary batteries A1-A4, B1, and B2 for testing were prepared and evaluated by a method below.
Constant current discharge of 2.5 mA at 25° C. was performed on the lithium primary batteries immediately after their assembly until the depth of discharge (DOD) becomes 90%
The batteries after Discharge 1 were placed in an environment at −30° C. Then, the batteries were discharged with pulse current of 300 mA for one second to measure battery voltage (closed circuit voltage)V1 after the pulse discharge.
In the preparation of the negative electrode, an Al foil with a thickness of 7 μm was attached to both surfaces of the Li foil in the negative electrode. Other conditions were equivalent to batteries A1-A4 to prepare and similarly evaluate lithium primary battery B3 for testing. In battery B3, the Li—Al alloy sheet of the negative electrode is pulverized to form a porous sheet.
In the preparation of the negative electrode, the molten liquid of Li and Al was kept in the mold at 150° C. for two hours, same as battery A2, to obtain the Li—Al alloy ingot. The ingot obtained was grounded to achieve an average particle size of 200 μm, and a grounded ingot was pressed into a sheet to obtain a porous negative electrode. Other conditions were equivalent to batteries Al to A4 to prepare and similarly evaluate lithium primary battery B4 for testing.
TABLE 1 shows evaluation results of voltage V1 after pulse discharge from DOD 90% in lithium primary batteries A1-A4 and B1-B4. Batteries A1-A4 are examples and batteries B1-B4 are comparative examples. Values of the Al content, Gα, Gβ, (Gα+Gβ)/2, and |Gα−Gβ|/(Gα+Gβ) of the Li—Al alloy sheet in each battery are indicated together with the evaluation results.
According to TABLE 1, batteries A1-A4 with (Gα+Gβ)/2 of 14 or less (average grain boundary number N of 8.75 or less) maintain voltage V1 of 1.8 V or more after pulse discharge under the environment of −30° C. even in a state of a DOD of 90%. High output characteristics at low temperatures is maintained.
In batteries B1-B2 with (Gα+Gβ)/2 exceeding 14, voltage V1 after pulse discharge has decreased to less than 1.8 V, and high output characteristics were not maintained at low temperatures. In batteries B3 and B4 including the porous Li—Al alloy sheet for the negative electrode, voltage V1 after pulse discharge has decreased to 1 V or less. The output characteristics at low temperatures have significantly decreased.
Since the grain sizes of the Li—Al alloy particles increase as (Gα+Gβ)/2 becomes smaller, the output characteristics improve. Accordingly, voltage V1 after pulse discharge is expected to increase. As illustrated in
In the preparation of the Li—Al alloy ingot, the molten liquid is necessarily cooled slowly, taking time, in order to achieve smaller (Gα+Gβ)/2. This deteriorates production efficiency. From the perspective of suppressing an increase of battery manufacturing costs, (Gα+Gβ)/2 is preferably 5 or more (average grain boundary number N of 3.12 or more).
In the preparation of the negative electrode, the extrusion speed at performing extrusion molding of the Li—Al alloy ingot was changed. Other conditions were equivalent to battery A3 to prepare and similarly evaluate lithium primary batteries A5 and A6 for testing.
In battery A5, the extrusion speed was changed from 50 m/minute to 150 m/minute. In battery A6, the extrusion speed was changed to 100 m/minute. The faster the extrusion speed is, the larger the difference between Gα and Gβ, resulting in larger grain boundary aspect ratio.
For batteries A3-A6, a pulse discharge test starting from DOD of 95% indicated below was performed in addition to the above pulse discharge test from DOD of 90%.
A constant current discharge of 2.5 mA at 25° C. was performed on the lithium primary batteries immediately after assembly until the DOD becomes 95% (Discharge 2).
The batteries after Discharge 2 were placed in the environment at −30° C. Then, the batteries were discharged with pulse current of 300 mA for one second to measure battery voltage (closed circuit voltage) V2 after pulse discharge.
TABLE 2 shows evaluation results. TABLE 2 shows values of the Al content, Gα, Gβ, (Gα+Gβ)/2, and |Gα−Gβ|/(Gα+Gβ) of the Li—Al alloy sheet in each battery together with the evaluation results. Voltages V1 of batteries A3 and A4 are the same as that in TABLE 1.
According to TABLE 2, all batteries A3-A6 maintain voltage V1 Of 1.8 V or more after pulse discharge from DOD of 90%. High output characteristics at low temperatures can be maintained. On the other hand, in batteries A5 and A6 with grain boundary aspect ratio A exceeding 0.2, voltage V2 after pulse discharge from DOD of 95% at −30° C. has dropped below 1.8V.
In contrast, in batteries A3 and A4 with grain boundary aspect ratio A of 0.2 or less, voltage V2 after pulse discharge at −30° C. was maintained at 1.8 V or more even in an extremely deep discharge state of DOD of 95%.
Batteries A7-A9 and B5 The Al content with respect to whole Li and Al in the Li—Al alloy ingot was changed from 0.3 mass %. Other conditions were equivalent to battery A4 to prepare lithium primary batteries A7-A9 for testing and voltage V1 and voltage V2 were similarly evaluated.
TABLE 3 shows evaluation results. TABLE 3 shows values of the Al content, Gα, Gβ, (Gα+Gβ)/2, and |Gα−Gβ|/(Gα+Gβ) of the Li—Al alloy sheet in each battery together with the evaluation results. To facilitate description, results of battery A4 were duplicated from TABLE 2.
According to TABLE 3, Gα and Gβ decrease as the Al content in the Li—Al alloy sheet decreases. As a result, (Gα+Gβ)/2 decreases but grain boundary aspect ratio A tends to increase. In battery A9 with Al content less than 0.3 mass %, grain boundary aspect ratio A exceeds 0.2. Although voltage V1 of 1.8 V or more was maintained after pulse discharge from DOD of 90%, voltage V1 of 1.8 V or more was not maintained after pulse discharge from DOD of 95%.
In battery B5 with Al content exceeding 1.0 mass %, (Gα+Gβ)/2 exceeds 14 and voltages V1 and V2 after pulse discharge were not maintained at 1.8 V or more.
A non-aqueous electrolyte battery according to the present disclosure has high energy density and good discharge characteristics, and is preferably applicable to, for example, main power sources of a range of meters and memory backup power sources.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-103581 | Jun 2021 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2022/018113 | 4/19/2022 | WO |
