JPWO2015056412A1 - Lithium ion secondary battery - Google Patents

Abstract

The positive electrode active material of the lithium ion secondary battery of the present invention includes a first active material, a second active material having an operating potential lower than the operating potential of the first active material and higher than the operating potential of the negative electrode active material, Have The first active material is made of a lithium transition metal oxide containing Li and at least one transition metal element among Ni, Co, and Mn. The second active material is made of a polyanionic material containing lithium and a transition metal element. The lithium ion secondary battery is configured to stop discharging at the operating potential of the first active material.

Description

  The present invention relates to a lithium ion secondary battery, and more particularly to a positive electrode active material.

Lithium ion secondary batteries are small and have high energy density, and are widely used as power sources for portable electronic devices. As the positive electrode active material, layered compounds such as LiCoO ₂ have been mainly used. However, these compounds have a drawback that oxygen is easily desorbed at about 150 ° C. in a fully charged state, and this easily causes an oxidative exothermic reaction of the non-aqueous electrolyte.

  Further, a high-capacity material such as Si-based material has been studied as a negative electrode active material. However, the irreversible capacity of these materials is larger than that of graphite-based negative electrode materials. For this reason, in the battery using the layered compound as the positive electrode and the Si-based material as the negative electrode, lithium of the layered compound is consumed by the irreversible capacity of the negative electrode, and the battery capacity is reduced.

In recent years, from the viewpoint of the amount of resources and environmental load, as the positive electrode active material, olivine phosphate-based compounds LiMPO ₄ (M = Fe, Mn and the like) and lithium silicate compounds Li ₂ MSiO ₄ (M = Fe, Mn and the like) are used. Proposed. In Patent Documents 1 and 2, it is considered that these materials are mixed with the layered compound to be used for irreversible capacity consumption of the negative electrode.

However, in LiMPO ₄ and Li ₂ MSiO ₄ , the characteristics of the material itself are greatly different. Therefore, unless the usage method as a battery is appropriately performed, the characteristics of the material cannot be utilized.

JP 2011-238490 A JP 2010-257592 A

  This invention is made | formed in view of this situation, and makes it a subject to provide the lithium ion secondary battery which can replenish the irreversible capacity | capacitance of a negative electrode and utilized the characteristic of material.

The lithium ion secondary battery of the present invention is a lithium ion secondary battery having a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, and an electrolyte solution,
The positive electrode active material includes a first active material, and a second active material having an operating potential lower than the operating potential of the first active material and higher than the operating potential of the negative electrode active material,
The first active material is composed of a lithium transition metal oxide containing Li and at least one transition metal element of Ni, Co, and Mn,
The second active material is made of a polyanionic material containing lithium and a transition metal element, and is configured to stop discharge at the operating potential of the first active material.

  According to the present invention, as the positive electrode active material, a first active material made of a lithium transition metal oxide and a second active material made of a polyanion material having a lower operating potential than the first active material are used. The discharge is stopped at the operating potential of one positive electrode active material. For this reason, the irreversible capacity | capacitance of a negative electrode can be replenished with a 2nd active material, and the lithium ion secondary battery which utilized the characteristic of the 1st active material can be provided.

It is explanatory drawing which shows the idea of the irreversible capacity | capacitance of the negative electrode of the lithium ion secondary battery of this invention, and the irreversible capacity | capacitance of a positive electrode. It is a figure which shows the charging / discharging curve of the battery 1, Comprising: A horizontal axis shows a capacity | capacitance and a vertical axis | shaft is a figure which shows the electric potential. The upper diagram in FIG. 3 is a charge / discharge curve when the battery 2 is charged / discharged between 3 V and 4.5 V, the horizontal axis indicates the capacity, and the vertical axis indicates the potential. 3 is a charge / discharge curve when the battery 2 is charged / discharged between 2.5 V and 4.5 V, and the horizontal axis indicates the capacity and the vertical axis indicates the potential. It is a figure which shows the charging / discharging curve of the battery 3, Comprising: A horizontal axis shows a capacity | capacitance and a vertical axis | shaft is a figure which shows the electric potential. It is a figure which shows the charging / discharging curve of the battery 1, Comprising: A horizontal axis shows a capacity | capacitance ratio and a vertical axis | shaft is a figure which shows the electric potential. It is a figure which shows the charging / discharging curve of the battery 4, Comprising: A horizontal axis shows a capacity | capacitance ratio and a vertical axis | shaft is a figure which shows the electric potential. It is a figure which shows the charging / discharging curve of the battery 5, Comprising: A horizontal axis shows capacity | capacitance ratio and a vertical axis | shaft is a figure which shows the electric potential. For batteries 1, 4, and 5, the discharge predicted when the irreversible capacity of the negative electrode is 50% of the charge capacity of the positive electrode active material (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ) The curve is shown. It is a figure which shows the discharge curve of the batteries 6-8. 3 is a diagram showing a charge / discharge curve of a battery 9. FIG.

  The detail of the lithium ion secondary battery which concerns on embodiment of this invention is demonstrated.

  As shown in FIG. 1, the positive electrode active material of the lithium ion secondary battery includes a first active material and a second active material. The first active material is a reversible positive electrode active material, and the second active material is a pre-doping positive electrode active material for replenishing the irreversible capacity of the negative electrode.

  That is, the positive electrode active material has a first active material and a second active material. The operating potential of the second active material is lower than the operating potential of the first active material and higher than the operating potential of the negative electrode active material. When the first active material, the second active material, and the negative electrode active material are each assembled with a secondary battery using lithium as a counter electrode, charge / discharge is performed to create a charge / discharge curve. After passing through a plateau portion where the ratio (ΔV / ΔC) of the potential change (ΔV) to (ΔC) is small, the ratio (ΔV / ΔC) tends to be larger than the plateau portion.

Here, the “operating potential” is formed by discharging the secondary battery when each of the first active material, the second active material, and the negative electrode active material is assembled with the counter electrode lithium metal. The potential relative to Li / Li ⁺ when the plateau portion of the discharge curve is switched to a portion where the ratio of potential change to capacity change is large.

  The operating potential of the second active material is lower than the operating potential of the first active material. For this reason, at the time of charge, mainly lithium ions of the second active material having a low operating potential are preferentially released. Lithium ions released from the second active material are occluded by the negative electrode active material. As charging progresses, the first active material then releases lithium ions. Thus, first, lithium ions released from the second active material are occluded in the negative electrode active material to replenish the irreversible capacity of the negative electrode active material. Therefore, after that, lithium ions released from the first active material are occluded in the reversible capacity of the negative electrode active material. For this reason, a 1st active material can be used for the reversible part of charging / discharging.

  The reversible capacity of the negative electrode active material corresponds to the initial discharge capacity of the negative electrode active material, and the irreversible capacity of the negative electrode active material corresponds to the initial charge capacity of the negative electrode active material minus the initial discharge capacity of the negative electrode active material. To do.

  In the lithium ion secondary battery of the present invention, the discharge is stopped at the operating potential of the first active material. Since the working potential of the second active material is lower than the working potential of the first active material, when the discharge is stopped at the working potential of the first active material in the lithium ion secondary battery, the capacity of the first active material is mainly expressed. The capacity of the second active material is hardly expressed. A lithium ion secondary battery that makes use of the characteristics of the first active material can be provided.

The operating potential of the first active material is such that the absolute value of the ratio (ΔV / ΔC) of the potential change (ΔV) to the capacity change (ΔC) of the first active material in the discharge curve of the first active material is 0.05V. It is preferable that the potential be at least / mAhg ⁻¹ . By using the first active material having such an operating potential as the positive electrode active material of the lithium ion secondary battery together with the second active material, the discharge voltage of the battery is in accordance with the discharge characteristics of the first active material from the start of discharge. Through the plateau portion, it suddenly increases at the operating potential. For this reason, the electric potential which should stop discharge can be detected correctly, and it becomes easy to control a battery.

  The first active material is made of a lithium transition metal oxide containing Li and at least one transition metal element among Ni, Co, and Mn. Lithium transition metal oxide is most practically used among current positive electrode active materials, has a high discharge voltage, and has low resistance. The potential in the discharge curve of the lithium transition metal oxide gradually decreases as the capacity increases.

  On the other hand, the second active material is made of a polyanionic material containing lithium and a transition metal. Many polyanionic materials have an operating potential lower than that of the lithium transition metal oxide. Moreover, many of the polyanion materials exhibit unique battery characteristics when they are included in the positive electrode active material to form a battery and charge and discharge are performed.

  Specifically, when a lithium ion secondary battery including a polyanion-based material in the positive electrode is formed and the battery is overdischarged exceeding the operating potential of the polyanion-based material, the potential rapidly decreases with the end potential as a boundary. For this reason, in the lithium ion secondary battery of the present invention, it is easy to determine whether or not to stop the discharge, and it is easy to determine the end of the discharge. It can prevent accidental overdischarge. The battery control method of the lithium transition metal oxide constituting the first active material can be used as it is.

  Since the polyanion material constituting the second active material does not use rare metal, it is inexpensive. The polyanion material is lighter than the lithium transition metal oxide constituting the first active material. Compared to the case where the positive electrode active material is composed of only the first active material, when the positive electrode active material is composed of the first active material and the second active material, an inexpensive and lightweight battery can be manufactured.

  The operating potential of the lithium transition metal oxide constituting the first active material is often 3 V or more, for example. In addition, the operating potential of the polyanionic material constituting the second active material is often less than 3V. Therefore, when discharge is performed on a battery having these as positive electrodes, the discharge is preferably stopped between 3 V and 3.7 V. Further, the discharge is preferably 3.2 V or more and 3.5 V or less. In this case, the discharge of the battery ends before the capacity of the second active material is developed. Therefore, a battery reflecting the battery characteristics of the first active material can be provided.

  The configuration for stopping the discharge of the lithium ion secondary battery at the operating potential of the first active material includes, for example, a voltage detection means and a circuit breaker. The voltage detection means detects the voltage change and the capacity change of the positive electrode, and when the ratio (ΔV / ΔC) becomes a predetermined value or more in absolute value, the circuit breaker stops the discharge.

  When the discharge is stopped at the operating potential of the first active material, the first active material preferably has an initial charge capacity equal to or greater than the reversible capacity of the negative electrode active material. This is because the reversible capacity of the negative electrode active material is supplemented with lithium ions released from the first active material. For this reason, the first active material may be contained in the positive electrode active material so as to have an amount of lithium corresponding to or greater than the reversible capacity of the negative electrode active material.

  When the discharge is stopped at the operating potential of the first active material, the second active material preferably has an initial charge capacity equal to or greater than the irreversible capacity of the negative electrode active material. This is because the irreversible capacity of the negative electrode active material is compensated with lithium ions released from the second active material. For this reason, the second active material may be included in the positive electrode active material so as to have an amount of lithium corresponding to or greater than the irreversible capacity of the negative electrode active material.

  When the discharge is stopped at the operating potential of the first active material, 0.9 <A / B <1.1, where A is the initial charge capacity of the second active material and B is the irreversible capacity of the negative electrode. It is preferable to have the following relationship. In this case, the second active material having a capacity substantially corresponding to the irreversible capacity of the negative electrode is added to the first active material, and most of the reversible capacity of the first active material is effectively utilized for the reversible capacity of the negative electrode. can do.

  Furthermore, when the discharge is stopped at the operating potential of the first active material, the total value of the initial charge capacity of the first active material and the initial charge capacity of the second active material is about the same as the initial charge capacity of the negative electrode active material. It is good to do.

The first active material is made of a lithium transition metal oxide containing Li and at least one transition metal element among Ni, Co, and Mn. As the lithium transition metal oxide, for example, a metal composite oxide of lithium and a transition metal such as a lithium / manganese composite oxide, a lithium / cobalt composite oxide, or a lithium / nickel composite oxide is used. Lithium transition metal oxide of the _{formula: LiNi 1-xy Co x Mn} y O 2 (0 ≦ x ≦ 1,0 ≦ y ≦ 1,0 ≦ 1-xy), or _{Formula: LiNi 2-xy Co x Mn} y O ₄ (0 ≦ x ≦ 2, 0 ≦ y ≦ 2, 0 ≦ 2-xy) is preferable. Specific examples of the first active material include LiCoO ₂ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiNi _0.5 Co _0.2 Mn _0.3 O ₂ , LiMn ₂ O ₄ , LiNi _{0. 5} Mn _1.5 O ₄ , LiNi _1-x Co _x O ₂ (0 ≦ x ≦ 1) and the like.

The second active material is made of a polyanionic material containing lithium (Li). The polyanionic material is LiMPO ₄ (where M is one or more selected from the group consisting of Ni, Co, Mn and Fe), Li ₂ MSiO ₄ (where M is Ni, Co, Mn and 1 or more selected from the group consisting of Fe), LiMBO ₃ (where M is one or more selected from the group consisting of Ni, Co, Mn and Fe), and Li ₂ FeP ₂ O ₇ It is preferable to consist of at least one selected from the group. The _inside, Li 2 MSiO ₄ C., and _further, it is Li 2 FeSiO _4. When a battery is manufactured using Li ₂ FeSiO ₄ as the second active material, the voltage gradually decreases as the capacity increases during discharge, and therefore the end voltage can be easily determined.

  The second active material is preferably formed by combining the polyanionic material and a conductive material. It is preferable that the polyanion-based material and the conductive material are mixed and bonded to each other in a very small nano-level fine particle state of 100 nm or less. As a result, the conductive paths between the second active materials are increased, and the second active material can be involved in the reaction, so that the utilization rate of the second active material is increased. Lithium ions are easily released during charging. As the conductive material, for example, a material having higher conductivity than the polyanionic material is used. For example, a carbon material is preferably used as the conductive material. As the carbon material, acetylene black (AB), ketjen black (KB), carbon black, carbon nanotube, graphene, carbon fiber, graphite, or the like can be used. Among these, acetylene black (AB), ketjen black (KB), and carbon black are preferable.

  In the second active material, the mass ratio of the conductive material when the polyanionic material is 100 parts by mass is preferably 2 parts by mass or more and 60 parts by mass or less, and more preferably 5 parts by mass or more and 30 parts by mass. It may be the following. In this case, the polyanion material and the conductive material are uniformly dispersed, and lithium ions can be greatly extracted during charging.

  Table 1 illustrates the operating potential of a material that can be a first active material and the operating potential of a material that can be a second active material. The first active material and the second active material are not limited to those listed in Table 1, and any material that can be applied within the scope of the present invention can be used.

The negative electrode has a negative electrode active material. The operating potential of the negative electrode active material is lower than that of the first active material and the second active material constituting the positive electrode active material. The operating potential of the negative electrode is often 0.5 V (vs Li / Li ⁺ ) or less in many cases. For example, when the negative electrode active material is made of SiOx, it is 0.4 V (vs Li / Li ⁺ ), and when the negative electrode active material is made of carbon, it is 0.2 V (vs Li / Li ⁺ ).

  The negative electrode active material preferably contains one or more elements selected from an element that can be alloyed with lithium, an elemental compound having an element that can be alloyed with lithium, and carbon. These have a relatively large irreversible capacity. As in the present invention, by replenishing these irreversible capacities with lithium ions released from the second active material, the lithium ions released from the first active material can be turned into the reversible capacity of the negative electrode active material. it can.

  Elements capable of alloying with lithium are Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Ti, Ag, Zn, Cd, Al, Ga, In, Si, Ge, It is good to consist of at least 1 sort (s) chosen from the group of Sn, Pb, Sb, and Bi. Among these, silicon (Si) or tin (Sn) is preferable.

  The elemental compound having an element capable of alloying with lithium is preferably a silicon compound or a tin compound. The silicon compound is preferably SiOx (0.5 ≦ x ≦ 1.5). Examples of the tin compound include tin alloys (Cu—Sn alloys, Co—Sn alloys, etc.), Sn oxides, and the like. Among these compounds, the ratio of irreversible capacity to the initial charge capacity is 40 to 50% for SiOx, 30 to 40% for tin alloy, and 40 for Sn oxide (SnO). -50%, both of which have a large irreversible capacity.

  Further, as the negative electrode active material, a carbon-based material such as metallic lithium or graphite, or an oxide material such as lithium titanate may be used.

  A positive electrode has said positive electrode active material. In many cases, the positive electrode is formed by coating the surface of the current collector with a positive electrode active material. The positive electrode can have the same structure as a normal positive electrode for a lithium ion secondary battery other than the positive electrode active material.

  For example, the positive electrode active material may include a conductive additive such as acetylene black (AB), ketjen black (KB), vapor grown carbon fiber (VGCF), or polyvinylidene fluoride (PVdF). Add a binder such as polytetrafluoroethylene (PTFE) or styrene-butadiene rubber (SBR), or a solvent such as N-methyl-2-pyrrolidone (NMP) to apply as a paste to the current collector Can produce a positive electrode. The amount of the conductive auxiliary agent used is not particularly limited, but for example, it can be 5 to 20 parts by mass with respect to 100 parts by mass of the positive electrode active material. Further, the amount of the binder used is not particularly limited, but may be 5 to 20 parts by mass with respect to 100 parts by mass of the positive electrode active material, for example. Further, as another method, a mixture of the positive electrode active material, the above-described conductive additive and binder is kneaded using a mortar or a press to form a film, and this is crimped to the current collector with a press. The positive electrode can also be manufactured by the method.

  A negative electrode has said negative electrode active material. In the negative electrode, the negative electrode active material often covers the current collector surface. The negative electrode is preferably applied to the surface of the current collector, together with the conductive auxiliary agent, the binder, and / or the solvent, as necessary, in the same manner as the above positive electrode active material.

  For both the positive electrode and the negative electrode, the current collector is not particularly limited, and conventionally used materials such as aluminum foil, aluminum mesh, and stainless steel mesh can be used. Furthermore, a carbon nonwoven fabric, a carbon woven fabric, etc. can be used as a collector.

  The shape and thickness of the positive electrode and the negative electrode are not particularly limited. For example, the positive electrode and the negative electrode are filled with a positive electrode active material and a negative electrode active material, and then compressed to have a thickness of 10 to 200 μm, more preferably 20 It is preferable to be set to ~ 100 μm.

As an electrolytic solution, a lithium salt such as lithium perchlorate, LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ or the like is added to a known non-aqueous solvent such as ethylene carbonate, dimethyl carbonate, propylene carbonate, dimethyl carbonate to 0.5 mol / L to 1 A solution dissolved at a concentration of 7 mol / L may be used, and other known battery components may be used.

  The lithium ion secondary battery can be mounted on a vehicle. The vehicle may be an electric vehicle or a hybrid vehicle. The lithium ion secondary battery may be connected to, for example, a traveling motor mounted on a vehicle and used as a drive source. Lithium ion secondary batteries can also be installed in electronic devices such as personal computers and portable communication devices.

(Battery 1)
The battery 1 is a secondary battery using a positive electrode active material made of LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (first active material).

As the first active material, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (NCM, manufactured by Hosen) was prepared. A first active material, a conductive additive and a binder were mixed to prepare a slurry. The conductive auxiliary agent is acetylene black (AB), and the binder is polyvinylidene fluoride (PVDF). These mixing mass ratios are positive electrode active material: AB: PVDF = 90: 2: 8 (mass%). The slurry was coated on an aluminum current collector and vacuum dried at 150 ° C. overnight. Thus, a positive electrode was obtained.

Metal Li was used as the negative electrode. The electrolytic solution was prepared by dissolving the electrolyte in a non-aqueous solvent. The non-aqueous solvent is obtained by mixing EC (ethylene carbonate) and DEC (diethyl carbonate) at a volume ratio of EC: DEC = 1: 1. The electrolyte consisting of LiPF _6. The concentration of LiPF ₆ in the electrolytic solution was 1 mol / liter. A coin cell was manufactured using the positive electrode, the negative electrode, and the electrolytic solution. The electrodes and batteries were produced in a glove box under an argon atmosphere.

A charge / discharge test was performed on the manufactured battery. The charging conditions were 4.5 V, constant current-constant voltage charging method (CCCV charging method), and 10 hours. The discharge conditions were 3V, constant current method (CC discharge method). The test was performed in an environment of 30 ° C. The charge / discharge test results are shown in FIG. FIG. 2 shows a charge / discharge curve of a first active material (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ) prepared using Li / Li ⁺ as a reference potential. The horizontal axis of FIG. 2 indicates the battery capacity, and the vertical axis indicates the battery potential (Li / Li ⁺ reference).

  For comparison with Examples 1 and 2, which will be described later, FIG. 5 shows a charge / discharge curve in FIG. 2 when the horizontal axis is converted into a capacity ratio (%). The capacity ratio (%) on the horizontal axis in FIG. 5 refers to the ratio of the discharge capacity to the charge capacity in the evaluation voltage range (3-4.5 V). The same applies to FIGS. 6 and 7 described later.

As shown in FIG. 2, LiNi _1/3 Co _1/3 Mn _1/3 O _{2 has} an initial charge capacity of 205 mAh / g, an initial discharge capacity (reversible capacity) of 185 mAh / g, and an irreversible capacity of 20 mAh / g. The potential was 3.6V. In the discharge curve of LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , there is a plateau part that maintains the potential from 4.5 V to 3.6 V when the capacity at the start of discharge is between 0 and 180 mAh / g, After the end point, a rapid potential drop was observed. The end point of the plateau part is the absolute value of the ratio (ΔV / ΔC) of the potential change (ΔV) to the capacity change (ΔC) of LiNi _1/3 Co _1/3 Mn _1/3 O ₂ after the end point of the plateau part. The value was 0.05 V / mAhg ⁻¹ .

(Battery 2)
The battery 2 is a secondary battery using a positive electrode active material made of Li ₂ FeSiO ₄ (second active material).

Li ₂ FeSiO ₄ was prepared as follows. Ferric oxide Fe ₂ O ₃ (high purity chemical Co., Ltd., purity 99.99%) 1.0 mol and lithium carbonate Li ₂ CO ₃ (Kishida Chemical Co., Ltd., purity 99.5%) 1.1 Mole, silica SiO ₂ (manufactured by Nippon Aerosil Co., Ltd.) 1.0 mol, dextrin hydrate (manufactured by Wako Pure Chemical Industries, Ltd.) 1.2 mol is wet-mixed by a bead mill (100 μm diameter ZrO ₂ beads) For 3.5 hours. The obtained mixed solution was spray-dried by a thermal spray apparatus. The treatment conditions were an inlet temperature of 200 ° C., an exhaust temperature of 91 ° C., a tower pressure of 4 kPa, and a supply air volume of 0.80 m ³ / min. Nozzle air (primary pressure 0.7 MPaG, flow rate 40 NL / min.), Liquid feed rate (flow rate 20 mL / min.). The obtained powder was heat-treated under the conditions of atmosphere CO ₂ : H ₂ (70:30 cc), temperature 700 ° C., and time 2 hours. After cooling to room temperature, X-ray diffraction (XRD) measurement of the treated product was performed. As a result, Li ₂ FeSiO ₄ belonging to monoclinic crystal and space group P2 _{1 / n} was compounded with carbon.

A secondary battery (battery 2) was fabricated using Li ₂ FeSiO ₄ combined with carbon as the second active material. A secondary battery was produced in the same manner as the battery 1 except that Li ₂ FeSiO ₄ was used. The produced battery 2 was subjected to a charge / discharge test under the same conditions as the battery 1. The test results are shown in the upper and lower views of FIG. The upper and lower diagrams in FIG. 3 show the charge / discharge curves of the second active material (Li ₂ FeSiO ₄ ) prepared using Li / Li ⁺ as the reference potential, and the upper diagram shows the 3V-4.5V charge / discharge curves. The figure below shows a 2.5V-4.5V charge / discharge curve. The horizontal axis in the upper and lower diagrams of FIG. 3 indicates the battery capacity, and the vertical axis indicates the battery potential (Li / Li ⁺ reference).

In the upper diagram of FIG. 3, the initial charge capacity of Li ₂ FeSiO ₄ is 115 mAh / g, the initial discharge capacity is 15 mAh / g, and the irreversible capacity is 100 mAh / g. In the lower diagram of FIG. 3, the initial charge capacity of Li ₂ FeSiO ₄ was 106 mAh / g, the initial discharge capacity was 79 mAh / g, the irreversible capacity was 27 mAh / g, and the operating potential was 2.7 V. Li ₂ FeSiO ₄ had a very small initial discharge capacity in the potential region of 3V to 4.5V compared to the initial charge capacity. Up to 3V, almost no capacity was developed. In the potential region of 2.5V-4.5V, when discharging from 3V to 2.5V, the capacity gradually developed rapidly. From this, it was found that Li ₂ FeSiO ₄ hardly occludes Li when discharged up to 3V.

(Battery 3)
The battery 3 is a secondary battery using a positive electrode active material made of LiFePO ₄ (second active material).

LiFePO ₄ was compounded with carbon in the same manner as Li ₂ FeSiO ₄ . LiFePO ₄ composited with carbon was used as the second active material to produce a secondary battery (battery 3). Battery 3 was made in the same manner as Battery 1 except that LiFePO ₄ was used. The produced battery 3 was subjected to a charge / discharge test under the same conditions as the battery 1. The test results are shown in FIG. FIG. 4 shows a charge / discharge curve of the second active material (LiFePO ₄ ) prepared using Li / Li ⁺ as a reference potential. The horizontal axis in FIG. 4 indicates the capacity of the battery, and the vertical axis indicates the battery potential (Li / Li ⁺ reference).

As shown in FIG. 4, LiFePO _{4 had} an initial charge capacity of 164 mAh / g, an initial discharge capacity of 159 mAh / g, an irreversible capacity of 5 mAh / g, and an operating potential of 3.4V. In the discharge curve of LiFePO ₄ , immediately after the start of discharge, a plateau portion with almost no potential change was formed, and the potential in the vicinity of 3.4 V was maintained. After the end point of the plateau part, a rapid potential drop was observed. After the end of the plateau part, the absolute value of the ratio (ΔV / ΔC) of the potential change (ΔV) to the capacitance change (ΔC) of LiFePO ₄ was 0.14 V / mAhg ⁻¹ .

(Battery 4)
In order to manufacture the battery 4, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (first active material) of the battery 1 and Li ₂ FeSiO ₄ (second active material) combined with the carbon of the battery 2 are used. Material) was mixed at a mass ratio of 67%: 33% to prepare a positive electrode active material. Using this positive electrode active material, a secondary battery (battery 4) was produced in the same manner as the battery 1. The produced battery 4 was subjected to a charge / discharge test under the same conditions as the battery 1. The test results are shown in FIG. FIG. 6 shows a charge / discharge curve of a second active material (Li ₂ FeSiO ₄ ) prepared using Li / Li ⁺ as a reference potential. Similar to FIG. 5, the horizontal axis of FIG. 6 indicates the capacity ratio (%), and the vertical axis indicates the battery potential (Li / Li ⁺ reference).

As shown in FIG. 6, a positive electrode active comprising LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (first active material) and Li ₂ FeSiO ₄ (second active material) complexed with carbon. Battery 4 using the material had an initial charge capacity of 238 mAh / g, an initial discharge capacity of 173 mAh / g, an irreversible capacity of 65 mAh / g, and an operating potential of 3.6V. In the discharge curve of the battery 4, there was a plateau portion between the potential 4.5 V and 3.6 V at the start of discharge, and after that end point, the potential dropped rapidly. After the end point of the plateau portion, the absolute value of the ratio (ΔV / ΔC) of the potential change (ΔV) to the capacity change (ΔC) of the battery 4 was 0.06 V / mAhg ⁻¹ . The charge curve and discharge curve of the battery 4 showed a behavior similar to that of the battery 1 using only LiNi _1/3 Co _1/3 Mn _1/3 O ₂ as the positive electrode active material. However, the discharge capacity of the battery 4 was reduced to 78% with respect to the discharge capacity of the battery 1. This decrease is considered to be due to the addition of 33% of Li ₂ FeSiO ₄ that exhibits almost no discharge capacity at 3.0 V with respect to 100% of the positive electrode active material.

  In Battery 4, the potential drop after the end point of the plateau portion of the discharge curve was remarkable. For this reason, the end of the discharge voltage can be accurately detected and is easy to control.

(Battery 5)
In order to manufacture the battery 5, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (first active material) of the battery 1 and LiFePO ₄ (second active material) composited with the carbon of the battery 2. Were mixed at a mass ratio of 67%: 33% to prepare a positive electrode active material. Using this positive electrode active material, a secondary battery (battery 5) was produced in the same manner as the battery 1. The produced battery 5 was subjected to a charge / discharge test under the same conditions as the battery 1. The test results are shown in FIG. FIG. 7 shows a charge / discharge curve of the second active material (LiFePO ₄ ) prepared using Li / Li ⁺ as a reference potential. Similarly to FIG. 5, the horizontal axis of FIG. 7 indicates the capacity ratio (%), and the vertical axis indicates the battery potential (Li / Li ⁺ reference).

As shown in FIG. 7, a positive electrode comprising LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (first active material) and LiFePO ₄ (second active material) combined with carbon of battery 2. Battery 5 using the active material had an initial charge capacity of 236 mAh / g, an initial discharge capacity (reversible capacity) of 223 mAh / g, and an operating potential of 3.4 V. In the discharge curve of the battery 5, the potential gradually dropped from the potential 4.5v at the start of discharge to 3.6V, and then the potential once dropped suddenly between 3.6 and 3.4V. However, after that, the capacity was maintained at 3.4 V up to a capacity of 60 to 93%, and the potential dropped rapidly again at 93% or more. Thus, in the battery 5, there was a two-step voltage drop between 3.6 and 3.4V. The battery 5 having such a discharge curve is difficult to accurately detect the end voltage of the discharge and is difficult to control the voltage.

  5 to 7, the capacity ratio (%) of the batteries 1, 4, and 5 when the potential was 3.5 V and 3.0 V was obtained. The results are shown in Table 2.

  As shown in Table 2, in the batteries 1 and 4, there was no significant difference in the capacity ratio between the potentials of 3.5V and 3.0V. On the other hand, the capacity ratio of the battery 5 greatly increased when the potential was 3.0V compared to 3.5V.

All of the positive electrode active materials of the battery 1 are made of LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (NCM, first active material). FIG. 8 shows that the irreversible capacity of the negative electrode was 50% of the initial charge capacity of LiNi _1/3 Co _1/3 Mn _1/3 O ₂ for the positive electrode active material used in batteries 1, 4, and 5. The expected value of the discharge curve is shown. This expected discharge curve was determined as follows. LiNi _1/3 Co _1/3 Mn _1/3 O ₂ when initial discharge curves of the batteries 1, 4, 5 (FIGS. 5, 6, 7) are taken as 100% of the initial charge capacity of the battery 1. (First active material) It was converted into a ratio (%) of capacity (mAh) per 1 g.

FIG. 8 is a diagram showing the initial discharge curve in this way in relation to the capacity ratio with respect to the mass of only LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (first active material) and the potential. .

As shown in FIG. 8, when the irreversible capacity of the negative electrode is 50% of the initial charge capacity of LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , the battery 1 (LiNi _1/3 Co The capacity ratio of _1/3 Mn _1/3 O ₂ ) was reduced to 50%. On the other hand, the capacity ratio of the battery 4 (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ and Li ₂ FeSiO ₄ ) was 69%. The capacity ratio of the battery 5 (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ and LiFePO ₄ ) was 63%.

  As shown in FIG. 8, the discharge curves of the batteries 1, 4, and 5 all have a relatively flat plateau portion that starts from the beginning of discharge, and the potential is rapidly decreased near the end point. In the battery 4, after passing through the plateau portion, the potential is rapidly lowered, so that the end voltage can be easily detected. By setting the operating potential in the vicinity of the inflection point where the potential suddenly drops through the plateau portion as the end potential, the end potential of the discharge voltage of the battery can be accurately controlled, and excessive discharge can be prevented. On the other hand, in the battery 5, there is a sudden potential drop once through the first plateau portion, but when the discharge is further advanced, the second plateau portion is developed. For this reason, the battery 5 cannot accurately detect the end potential and may cause excessive discharge.

(Battery 6)
The battery 6 is a secondary battery using a positive electrode active material made of LiMn ₂ O ₄ (first active material). Other configurations are the same as those of the battery 1. The battery 6 was charged and discharged in the same manner as the battery 1, and the initial discharge curve is shown in FIG.

As shown in FIG. 9, LiMn ₂ O _{4 had} an initial charge capacity of 120 mAh / g, an initial discharge capacity (reversible capacity) of 110 mAh / g, an irreversible capacity of 10 mAh / g, and an operating potential of about 2.8V. In the discharge curve of LiMn ₂ O ₄ , there is a plateau portion of potential 4.15 V to 3.9 V between 10 and 120 mAh / g after the start of discharge, and after that end point, from 3.9 V to about 3.9 V at 120 mAh / g. A rapid potential drop was observed up to 2.8V. After the end point of the plateau portion, the absolute value of the ratio (ΔV / ΔC) of the potential change (ΔV) to the capacity change (ΔC) of LiMn ₂ O ₄ was 0.06 V / mAhg ⁻¹ . When the potential dropped to 2.8 V, a second plateau portion was developed that maintained about 2.8 V between 120 mAh / g and 220 mAh / g.

When LiMn ₂ O ₄ is overdischarged, the capacity increases from about 2.8V. This is considered to be because LiMn ₂ O ₄ was changed to Li ₂ Mn ₂ O ₄ . When it changes to Li ₂ Mn ₂ O ₄ , the cycle characteristics of the battery deteriorate.

Considering the discharge curve when LiFePO ₄ is added to LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (first active material) to form a positive electrode active material (FIG. 7), LiMn ₂ O ₄ is combined with LiFePO ₄ . _{When 4} is added, as shown by the one-dot chain line in FIG. 9, the discharge capacity increases while maintaining at about 3.4V. In the case of overdischarge, it is presumed that the cycle is deteriorated by suddenly decreasing to 2.8 V and changing LiMn ₂ O ₄ to Li ₂ Mn ₂ O ₄ .

Considering from the discharge curve (FIG. 8) when Li ₂ FeSiO ₄ is added to LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (first active material) to form a positive electrode active material, LiMn ₂ O ₄ In the case of adding Li ₂ FeSiO ₄ to, as indicated by the two-dot chain line in FIG. 9, it is estimated that the capacity increase is small from about 3.4 V and the capacity increases after 3 V or less. By adding Li ₂ FeSiO ₄ to LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , it is possible to prevent a potential drop to 2.8 V abruptly and to suppress deterioration of LiMn ₂ O ₄ and cycle deterioration. it can.

(Battery 7)
The battery 7 is a secondary battery using a positive electrode active material made of LiNi _0.5 Co _0.5 O ₂ (first active material). Other configurations are the same as those of the battery 1. The battery 7 was charged and discharged in the same manner as the battery 1, and the initial discharge curve is shown in FIG.

As shown in FIG. 9, the initial charge capacity of LiNi _0.5 Co _0.5 O ₂ (x = 0.5) is 180 mAh / g, the initial discharge capacity is 150 mAh / g, the irreversible capacity is 30 mAh / g, and the operating potential is about It was 3.5V. In the discharge curve of LiNi _0.5 Co _0.5 O ₂ , there is a plateau portion of potential 4.2 V to 3.5 V between 0 to 140 mAh / g after the start of discharge, and after that end point, 140 to 150 mAh. / G showed a rapid potential drop. After the end of the plateau part, the absolute value of the ratio (ΔV / ΔC) of the potential change (ΔV) to the capacity change (ΔC) of LiNi _0.5 Co _0.5 O ₂ is 0.05 V / mAhg ⁻¹ . there were.

Even when LiFePO ₄ or Li ₂ FeSiO ₄ is added to LiNi _0.5 Co _0.5 O ₂ , the same discharge curve as when LiFePO ₄ or Li ₂ FeSiO ₄ is added to LiMn ₂ O ₄ is drawn. It is estimated that

(Battery 8)
The battery 8 is a secondary battery using a positive electrode active material made of LiCoO ₂ (first active material). Other configurations of the battery 8 are the same as those of the battery 1. The battery 8 was charged / discharged in the same manner as the battery 1, and the initial discharge curve is shown in FIG.

As shown in FIG. 9, the initial charge capacity of LiCoO ₂ was 150 mAh / g, the initial discharge capacity was 140 mAh / g, the irreversible capacity was 10 mAh / g, and the operating potential was about 3.8V. In the discharge curve of LiCoO ₂ , there is a plateau portion of potential 4.2 V to 3.9 V between 0 and 130 mAh / g after the start of discharge, and after that end point, a rapid potential drop occurs at 130 to 140 mAh / g. Indicated. After the end point of the plateau portion, the absolute value of the ratio (ΔV / ΔC) of the potential change (ΔV) to the capacity change (ΔC) of LiCoO ₂ was 0.06 V / mAhg ⁻¹ .

Even when LiFePO ₄ or Li ₂ FeSiO ₄ is added to LiCoO ₂ , it is presumed that a discharge curve similar to that when LiFePO ₄ or Li ₂ FeSiO ₄ is added to LiMn ₂ O ₄ is drawn.

(Battery 9)
The battery 9 is a secondary battery using a positive electrode active material made of LiFeBO ₃ (second active material). Other configurations of the battery 9 are the same as those of the battery 1. The battery 9 was charged and discharged in the same manner as the battery 1, and the charge / discharge curve is shown in FIG.

As shown in FIG. 10, LiFeBO _{3 had} an initial charge capacity of 275 mAh / g, an initial discharge capacity of 215 mAh / g, an irreversible capacity of 60 mAh / g, and an operating potential of about 2.5V. In the discharge curve of LiFeBO ₃ , the capacity hardly increased from immediately after the start of discharge to 3V. A capacity of 40-100 mAh / g was observed between 3V and 2.5V.

When LiFeBO ₃ is added to the first active material, the discharge capacity hardly increases if the potential is up to 3 V during discharge. For this reason, the irreversible capacity | capacitance of a negative electrode can fully be supplemented by using it as a positive electrode active material with the 1st active material which has an operating potential of 3V or more.

(Battery 10)
The battery 10 is a battery manufactured by using the positive electrode and the electrolytic solution of the battery 4 and changing to a negative electrode containing SiO. In order to produce a negative electrode, a commercially available SiO powder was put in a ball mill, milled at 450 rpm for 20 hours in an Ar atmosphere, and then heated at 900 ° C. for 2 hours in an inert gas atmosphere. Processed. Thereby, SiO powder was disproportionated and disproportionated SiOx powder was obtained. When this disproportionated SiOx powder was subjected to X-ray diffraction (XRD) measurement using CuKα, a specific peak derived from simple silicon (Si) and silicon dioxide (SiO ₂ ) was confirmed. From this, it was found that elemental silicon and silicon dioxide were formed in the disproportionated SiOx powder. This disproportionated SiOx powder, a conductive additive, and polyimide (PI) as a binder were mixed, and a solvent was added to obtain a slurry-like mixture. Acetylene black (AB) was used as the conductive assistant. The solvent was NMP. The mass ratio of the disproportionated SiOx powder, the conductive auxiliary agent, and the binder was in percentage, and the disproportionated SiOx powder / conductive auxiliary agent / binder was 80/2/18. Next, the slurry-like mixture was formed into a film on one side of a copper foil as a current collector using a doctor blade. After film formation, the film was pressed at a predetermined pressure, heated (200 ° C., 2 hours), and allowed to cool. As a result, a negative electrode in which a negative electrode material (negative electrode active material layer) was fixed on the current collector surface was produced.

The ratio of the irreversible capacity to the total of the total reversible capacity and irreversible capacity of SiO is 43%. In order to supplement the irreversible capacity of the negative electrode active material in a considerable proportion as described above, the second active material and the negative electrode are adjusted to have the following relationship. When the initial charge capacity of the second active material (Li ₂ FeSiO ₄ ) is A and the irreversible capacity of the negative electrode is B, the relationship is 0.9 <A / B <1.1. This volume _is a value in the case of stopping the discharge of the battery in LiNi _1/3 Co _{1/3 Mn} 1/3 _{O 2} operating potential (first active material) (3.6V). The mass ratio of the positive electrode active material to the negative electrode active material included in the battery 10 is, for example, 100 mass parts of SiO as the negative electrode active material so that the capacity of the positive electrode active material and the negative electrode has the above relationship. In this case, the first active material of the positive electrode active material is 810 parts by mass, and the second active material is 810 parts by mass. The second active material having a capacity substantially corresponding to the irreversible capacity of the negative electrode is added to the first active material, and most of the reversible capacity of the first active material can be effectively utilized for the reversible capacity of the negative electrode.

Claims (7)

A lithium ion secondary battery having a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, and an electrolyte solution,
The positive electrode active material includes a first active material, and a second active material having an operating potential lower than the operating potential of the first active material and higher than the operating potential of the negative electrode active material,
The first active material is composed of a lithium transition metal oxide containing Li and at least one transition metal element of Ni, Co, and Mn,
The second active material is made of a polyanionic material containing lithium and a transition metal element,
The lithium ion secondary battery is configured to stop discharging at the operating potential of the first active material. The operating potential of the first active material is such that the absolute value of the ratio of the potential change (ΔV) to the capacity change (ΔC) of the first active material is 0.05 V / mAhg ⁻¹ or more in the discharge curve of the first active material. The lithium ion secondary battery according to claim 1, wherein the potential is:   When the discharge is stopped at the operating potential of the first active material, 0.9 <A / B <1.1, where A is the initial charge capacity of the second active material and B is the irreversible capacity of the negative electrode. The lithium ion secondary battery of Claim 1 or 2 which has the relationship of these. The polyanionic material is LiMPO ₄ (where M is one or more selected from the group consisting of Ni, Co, Mn and Fe), Li ₂ MSiO ₄ (where M is Ni, Co, Mn). And at least one selected from the group consisting of Fe and Fe), LiMBO ₃ (wherein M is at least one selected from the group consisting of Ni, Co, Mn and Fe), and Li ₂ FeP ₂ O ₇ The lithium ion secondary battery according to claim 1, comprising at least one selected from the group consisting of: The lithium transition metal oxide has the formula: LiNi _1-xy Co _x Mn _y O ₂ (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ 1-xy), or the formula: LiNi _2-xy Co _x Mn _{y O 4 (0 ≦ x ≦} 2,0 ≦ y ≦ 2,0 ≦ 2-xy) lithium ion secondary battery according to claim 1 which is represented by.   6. The negative electrode active material according to claim 1, wherein the negative electrode active material includes at least one element selected from an element that can be alloyed with lithium, an elemental compound having an element that can be alloyed with lithium, and carbon. Lithium ion secondary battery.   The lithium ion secondary battery according to claim 6, wherein the elemental compound having an element capable of alloying with lithium is a silicon compound or a tin compound.

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