JP6575316B2 - Lithium ion secondary battery charge / discharge method, battery module, and lithium ion secondary battery - Google Patents

Description

  The present invention relates to a charge / discharge method for a lithium ion secondary battery, a battery module, and a lithium ion secondary battery.

  Lithium ion secondary batteries are widely used in electronic devices such as personal computers and communication terminals, automobiles and the like because of their high energy density.

  The lithium ion secondary battery generally has a pair of electrodes in which an active material layer is laminated on a current collecting layer such as a metal foil, and these are electrically separated by a separator, and lithium ions are provided between the negative electrode and the positive electrode. It is comprised so that it may charge / discharge by performing delivery.

  In such a lithium ion secondary battery, it is known to add tungsten (W) to the positive electrode active material in order to improve output characteristics (see, for example, Japanese Patent Application Laid-Open No. 2001-6676).

Japanese Patent Laid-Open No. 2001-6676

  However, when the inventors conducted a test on a lithium ion secondary battery in which tungsten was added to the positive electrode active material, it was found that although the output characteristics were excellent in the initial stage, the capacity was significantly reduced when the charge / discharge cycle was repeated. . That is, the conventional lithium ion secondary battery to which tungsten is added has a disadvantage that the capacity retention rate in the charge / discharge cycle is small. The present invention has been made based on the above-described circumstances, and provides a charge / discharge method for a lithium ion secondary battery, a battery module, and a lithium ion secondary battery that can significantly increase the capacity maintenance rate in a charge / discharge cycle. With the goal.

  The invention made to solve the above-mentioned problems is a method for charging and discharging a lithium ion secondary battery comprising a positive electrode having a positive electrode active material, wherein the positive electrode active material contains tungsten, and the lower limit potential of the positive electrode is reduced to lithium. It is characterized by being 3.7 V or more on the basis.

  Another invention made to solve the above problems is a battery module comprising a lithium ion secondary battery including a positive electrode having a positive electrode active material, and a mechanism for controlling charge and discharge of the lithium ion secondary battery. The positive electrode active material contains tungsten, and the control mechanism controls the lower limit potential of the positive electrode to be 3.7 V or higher with respect to lithium.

  Yet another invention made to solve the above-mentioned problems is a lithium ion secondary battery comprising a positive electrode having a positive electrode active material and a negative electrode having a negative electrode active material, wherein the positive electrode active material contains tungsten. The negative electrode active material contains graphite, and the lowest ultimate potential of the positive electrode is 3.7 V or more based on lithium.

  Here, the “minimum reached potential of the positive electrode” means the positive electrode potential when the negative electrode potential becomes 1.0 V with respect to lithium in the discharge of the secondary battery.

  The charge / discharge method, the battery module, and the lithium ion secondary battery of the lithium ion secondary battery of the present invention can remarkably increase the capacity maintenance rate in the charge / discharge cycle of the lithium ion secondary battery.

FIG. 1 is a cross-sectional view of a battery.

  Hereinafter, embodiments of a charge / discharge method for a lithium ion secondary battery, a battery module, and a lithium ion secondary battery according to the present invention will be described in detail.

[Lithium ion secondary battery]
First, a lithium ion secondary battery used in the charge / discharge method of the present invention will be described. The lithium ion secondary battery mainly includes a power generation element in which a positive electrode having a positive electrode active material and a negative electrode having a negative electrode active material are alternately superimposed via a separator. The power generating element is housed in a case, and the case is filled with an electrolytic solution. Note that the lithium ion secondary battery may include a reference electrode for measuring the positive electrode potential.

<Positive electrode>
The positive electrode has a current collecting layer and a positive electrode active material layer. The positive electrode may have an intermediate layer between the current collecting layer and the positive electrode active material layer.

(Collector layer)
The current collecting layer is a conductive layer. As the material for the current collecting layer, metals such as aluminum, copper, iron, nickel, or alloys thereof are used. Among these, aluminum, an aluminum alloy, copper, and a copper alloy are preferable from the balance between high conductivity and cost, and aluminum and an aluminum alloy are more preferable. Moreover, foil, a vapor deposition film, etc. are mentioned as a formation form of a current collection layer, and foil is preferable from the surface of cost. That is, an aluminum foil is preferable as the current collecting layer of the positive electrode. Examples of aluminum or aluminum alloy include A1085P and A3003P defined in JIS-H-4000 (2014).

  The lower limit of the average thickness of the current collecting layer is preferably 5 μm and more preferably 10 μm. On the other hand, the upper limit of the average thickness of the current collecting layer is preferably 50 μm, and more preferably 40 μm. When the average thickness of the current collecting layer is smaller than the lower limit, the strength of the current collecting layer is insufficient, and it may be difficult to form an electrode. On the other hand, when the average thickness of the current collecting layer exceeds the above upper limit, the thickness of the other components may be insufficient to keep the thickness of the secondary battery constant. The “average thickness” means an average value of thicknesses measured at arbitrary ten points. In the following description, the term “average thickness” is defined similarly for other members.

(Middle layer)
The intermediate layer is a coating layer on the surface of the current collecting layer, and reduces the contact resistance between the current collecting layer and the positive electrode active material layer by containing conductive particles such as carbon particles. The configuration of the intermediate layer is not particularly limited, and can be formed by a composition containing, for example, a resin binder, conductive particles, and non-conductive inorganic particles. “Conductive” means that the volume resistivity measured in accordance with JIS-H-0505 (1975) is 10 ⁷ Ω · cm or less, and “nonconductive” It means that the volume resistivity is more than 10 ⁷ Ω · cm.

(Positive electrode active material layer)
The positive electrode active material layer is formed from a so-called positive electrode mixture containing a positive electrode active material. In addition, the positive electrode mixture forming the positive electrode active material layer includes optional components such as a conductive agent, a binder (binder), a thickener, and a filler as necessary.

As the positive electrode active material contained in the positive electrode active material layer, a powder of an active material capable of inserting and extracting lithium ions is used. Specific active materials include general formula Li _{1 + α} M1 _1-α O ₂ (0 ≦ α ≦ 0.2, M1 is Ni, Mn, Ti, Cr, Fe, Co, Cu, Zn, Al, Ge, Sn , At least one metal selected from Mg, Mo and Zr), a general formula Li _{1 + β} M2 _2-β O ₄ (0 ≦ β ≦ 0.5, M2 represents Ni, Mn, Ti, Cr, A compound represented by at least one metal selected from Fe, Co, Cu, Zn, Al, Ge, Sn, Mg, Mo, and Zr). Of the compounds represented by the general formula Li _{1 + α} M1 _1-α O ₂ , M1 preferably contains Ni, and M1 contains Ni, Mn and Co. Li _{1 + α} Ni _x Mn _y A compound represented by Co _z O ₂ (x + y + z = 1−α, 0 <x <1, 0 <y <1, 0 <z <1) is more preferable. In addition, you may use a positive electrode active material in mixture of 2 or more types mentioned above.

  The positive electrode active material preferably has a layered structure, and a lithium-containing transition metal oxide having a layered structure is particularly preferable. Moreover, nickel is preferable as the transition metal of the lithium-containing transition metal oxide. Furthermore, the positive electrode active material more preferably contains cobalt and manganese as transition metals of the lithium-containing transition metal oxide. As described above, by using a lithium ion secondary battery in which the positive electrode active material contains nickel, preferably further cobalt and manganese, the effect of improving the capacity retention rate in the charge / discharge method of the lithium ion secondary battery can be promoted.

In the lithium ion secondary battery used in the present invention, the positive electrode active material contains tungsten. This tungsten coats the surface of the lithium-containing transition metal oxide in the state of an oxide (WO ₃ ), for example. As described above, when the positive electrode active material contains tungsten, side reactions of the positive electrode active material can be suppressed, and lithium conductivity can be improved. As a result, the output characteristics of the lithium ion secondary battery can be improved. As a form containing tungsten, there is a method of coating the surface of particles of the positive electrode active material with a compound containing tungsten. In addition, there is a method in which a compound containing tungsten is supported on the particle surface of the positive electrode active material by dissolving the tungsten in a solid solution at the time of synthesis of the positive electrode active material and baking. In addition to tungsten, zirconium (Zr) may be further added.

  As a minimum of content of tungsten in the whole positive electrode active material, 0.05 mass% is preferred, 0.1 mass% is more preferred, and 0.5 mass% is still more preferred. On the other hand, the upper limit of the tungsten content is preferably 3% by mass, more preferably 2% by mass, and even more preferably 1% by mass. If the tungsten content is less than the lower limit, the effect of improving the output characteristics may be insufficient. On the other hand, when the tungsten content exceeds the above upper limit, the electron conductivity of the positive electrode active material is lowered, and the electric capacity of the lithium ion secondary battery may be lowered.

  Although it does not specifically limit as an average particle diameter of a positive electrode active material, For example, it is 0.1 micrometer or more and 20 micrometers or less. When the average particle size of the positive electrode active material is smaller than the above lower limit, the production and handling of the positive electrode active material may be difficult. Conversely, when the average particle size of the positive electrode active material exceeds the upper limit, the electronic conductivity of the active material layer may be reduced. Here, the “average particle diameter” is based on JIS-Z-8815 (2013), based on the particle size distribution measured by a laser diffraction / scattering method with respect to a diluted solution obtained by diluting particles with a solvent. It means a value at which the volume-based cumulative distribution calculated according to Z-8819-2 (2001) is 50%.

  As a minimum of content of the positive electrode active material in a positive electrode active material layer, 50 mass% is preferable, 70 mass% is more preferable, and 80 mass% is further more preferable. On the other hand, as an upper limit of content of a positive electrode active material, 99 mass% is preferable and 95 mass% is more preferable. By setting the content of the positive electrode active material particles in the above range, the electric capacity of the lithium ion secondary battery can be increased.

(Optional component)
The conductive agent is not particularly limited as long as it is a conductive material that does not adversely affect battery performance. Examples of such a conductive agent include carbon black such as natural or artificial graphite, furnace black, acetylene black, and ketjen black, metals, and conductive ceramics. Examples of the shape of the conductive agent include powder and fiber.

  The lower limit of the content of the conductive agent in the positive electrode active material layer is preferably 1% by mass, and more preferably 3% by mass. On the other hand, as an upper limit of content of a electrically conductive agent, 10 mass% is preferable and 9 mass% is more preferable. By setting the content of the conductive agent in the above range, the electric capacity of the lithium ion secondary battery can be increased.

  Examples of the binder include thermoplastic resins such as fluororesin (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc.), polyethylene, polypropylene; ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene. Examples thereof include elastomers such as butadiene rubber (SBR) and fluororubber; polysaccharide polymers.

  As a minimum of content of a binder in a cathode active material layer, 1 mass% is preferred and 3 mass% is more preferred. On the other hand, as an upper limit of content of a binder, 10 mass% is preferable and 9 mass% is more preferable. By making the content of the binder in the above range, the active material can be stably held.

  Examples of the thickener include polysaccharide polymers such as carboxymethylcellulose and methylcellulose. When the thickener has a functional group that reacts with lithium, it is preferable to deactivate this functional group in advance by methylation or the like.

  The filler is not particularly limited as long as it does not adversely affect battery performance. Examples of the main component of the filler include polyolefins such as polypropylene and polyethylene, silica, alumina, zeolite, glass, and carbon.

<Negative electrode>
The negative electrode has a current collecting layer and a negative electrode active material layer. Moreover, the negative electrode may have an intermediate | middle layer between a current collection layer and a negative electrode active material layer similarly to a positive electrode. This intermediate layer can have the same structure as the intermediate layer of the positive electrode.

(Collector layer)
The current collecting layer can have the same configuration as the current collecting layer of the positive electrode, but the material is preferably copper or a copper alloy. That is, copper foil is preferable as the current collecting layer of the negative electrode. Examples of the copper foil include rolled copper foil and electrolytic copper foil.

(Negative electrode active material layer)
The negative electrode active material layer is formed from a so-called negative electrode mixture containing a negative electrode active material. Moreover, the negative electrode composite material which forms a negative electrode active material layer contains arbitrary components, such as a electrically conductive agent, a binder (binder), a thickener, and a filler, as needed. The same components as those for the positive electrode active material layer can be used as optional components such as a conductive agent, a binder, a thickener, and a filler.

  As the negative electrode active material, a material capable of inserting and extracting lithium ions is used. Specific examples of the negative electrode active material include metals such as lithium and lithium alloys (lithium-aluminum, lithium-lead, lithium-tin, lithium-aluminum-tin, lithium-gallium, etc.); metal oxides; polyphosphate compounds; Examples thereof include carbon materials such as graphite and amorphous carbon (easily graphitized carbon or non-graphitizable carbon).

  Among these, graphite and amorphous carbon are preferable. By using graphite or amorphous carbon as the negative electrode active material, the irreversible capacity of the negative electrode can be increased. When the irreversible capacity of the negative electrode is larger than that of the positive electrode, the end of discharge of the battery is mainly determined by a change in the negative electrode potential. That is, in a battery using graphite or amorphous carbon, the negative electrode potential rapidly rises at the end of discharge and the discharge ends. Since the negative electrode potential rapidly rises at the end of discharge and discharge is terminated, the lower limit potential of the positive electrode can be maintained high. Moreover, the control which makes the minimum electric potential of the positive electrode at the time of discharge of a lithium ion secondary battery more than a fixed value can be facilitated.

  In addition, the negative electrode active material may be used by mixing two or more kinds described above, and it is particularly preferable to use a combination of graphite and non-graphitizable carbon. When graphite is used as the negative electrode active material, the irreversible capacity can be increased, but the output resistance increases as the battery is used due to the formation of the solid electrolyte interface (SEI) in the negative electrode. In contrast, non-graphitizable carbon does not contribute to an increase in output resistance because its irreversible capacity is mainly due to lithium trapping. Therefore, a mixture of graphite and non-graphitizable carbon can be used to suppress an increase in output resistance while increasing the irreversible capacity. Further, as a technique for increasing the irreversible capacity of the negative electrode, a method of mixing graphite and a sulfur-based material (SiO, Si—O—C) can also be employed.

  As a minimum of content of a negative electrode active material in a negative electrode active material layer, 60 mass% is preferred, 80 mass% is more preferred, and 90 mass% is still more preferred. On the other hand, as an upper limit of content of a negative electrode active material, 99 mass% is preferable and 98 mass% is more preferable. By setting the content of the negative electrode active material particles in the above range, the electric capacity of the lithium ion secondary battery can be increased.

  The lower limit of the irreversible capacity of the negative electrode active material layer is preferably 10 mAh / g, and more preferably 20 mAh / g. On the other hand, the upper limit of the irreversible capacity of the negative electrode active material layer is preferably 100 mAh / g, more preferably 80 mAh / g. By making the irreversible capacity | capacitance of a negative electrode active material layer more than the said minimum, the control which makes the minimum electric potential of the positive electrode at the time of discharge of a lithium ion secondary battery more than a fixed value can be facilitated. On the other hand, when the irreversible capacity of the negative electrode active material layer exceeds the above upper limit, the capacity of the lithium ion secondary battery may be insufficient.

<Separator>
As the material of the separator, for example, woven fabric, non-woven fabric, porous resin film or the like is used. Among these, a porous resin film is preferable. The main component of the porous resin film is preferably a polyolefin such as polyethylene or polypropylene from the viewpoint of strength. Moreover, you may use the porous resin film which compounded these resins and resin, such as an aramid and a polyimide.

<Electrolyte>
As the electrolytic solution, a known electrolytic solution usually used for a lithium ion secondary battery can be used. For example, cyclic carbonate such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), or diethyl carbonate (DEC). ), A solution in which lithium hexafluorophosphate (LiPF ₆ ) or the like is dissolved in a solvent containing a chain carbonate such as dimethyl carbonate (DMC) or ethyl methyl carbonate (EMC) can be used.

[Charging / discharging method of lithium ion secondary battery]
The method for charging and discharging the lithium ion secondary battery includes the step of charging the lithium ion secondary battery using the lithium ion secondary battery described above, and the step of discharging the lithium ion secondary battery, and a discharging step. The lower limit potential of the positive electrode is 3.7 V or higher with respect to lithium.

  The present inventors diligently studied the relationship between the lower limit potential and the capacity retention rate when using a lithium ion secondary battery in which the positive electrode active material contains tungsten (during charge / discharge cycle). It has been found that when the voltage is less than 3.7 V on the basis of lithium, the capacity retention rate is extremely lowered as the cycle progresses. The reason for this is not clear, but when the positive electrode potential is less than 3.7 V with respect to lithium, the tungsten oxide layer on the surface of the positive electrode active material becomes thick, or a tungsten compound with low lithium conductivity is generated. It is estimated that a high resistance layer is formed on the surface of the positive electrode active material due to the above. It is assumed that this high resistance layer increases the polarization at the time of charging and discharging and greatly reduces the capacity retention rate.

  Therefore, in the charging / discharging method of the lithium ion secondary battery, by setting the lower limit potential of the positive electrode to 3.7 V or more on the basis of lithium, the positive electrode active material contains tungsten, so that the lithium ion secondary battery is excellent in output characteristics. On the other hand, the capacity maintenance rate in the charge / discharge cycle can be remarkably increased.

[Battery module]
The battery module includes the lithium ion secondary battery and a mechanism for controlling charging / discharging of the lithium ion secondary battery, and the control mechanism sets the lower limit potential of the positive electrode to 3.7 V or more based on lithium. Control.

  As a specific method for controlling the lower limit potential of the positive electrode, for example, the discharge end potential (lower limit potential) of the positive electrode is set to 3.7 V or more, the positive electrode potential is monitored during discharge, and the positive electrode potential becomes the discharge end potential. It is possible to use a method of terminating the discharge of the battery at the point of time. In the case of this method, the control mechanism includes a monitoring unit that monitors the positive electrode potential and a discharge control unit that terminates the discharge. As a method for monitoring the positive electrode potential, a method of measuring the positive electrode potential based on the potential of the reference electrode provided in the lithium ion secondary battery, knowing in advance the relationship between the battery voltage and the positive electrode potential, and measuring the battery voltage. Examples thereof include a method for calculating a corresponding positive electrode potential, a method combining these, and the like.

  As a configuration of the discharge control section for terminating the discharge, for example, a configuration including an inverter that turns off the discharge cutoff switch when the output becomes low, or turns off the discharge cutoff switch when the positive electrode potential reaches a certain value. Examples include a configuration including a microcomputer having a program.

  When the discharge of the battery is terminated when the positive electrode potential reaches the discharge end potential, the positive electrode potential can be lower than 3.7 V depending on the control time lag. Therefore, a threshold potential higher than 3.7 V may be determined, and the battery discharge may be terminated when the threshold potential is reached. As this threshold potential, for example, a potential that is 0.1 V higher than the lower limit potential on the basis of lithium can be used.

  Even if the discharge end potential of the positive electrode is set to 3.7 V or higher, the positive electrode potential may become lower than 3.7 V due to sudden large current discharge or the like. However, unless the cycle in which the positive electrode potential is 3.7 V is repeated, the capacity retention rate does not decrease and the effect of the present invention is not hindered. That is, contrary to the setting of the discharge end potential of the positive electrode, an event in which the positive electrode potential becomes lower than 3.7 V is allowed in the gist of the invention.

  In the battery module, by controlling the lower limit potential of the positive electrode to 3.7 V or more on the basis of lithium, the capacity maintenance rate in the charge / discharge cycle of the lithium ion secondary battery in which the positive electrode active material contains tungsten can be significantly increased.

  Note that, in the case of a lithium ion secondary battery in which the minimum potential of the positive electrode is 3.7 V or more based on lithium due to the selection of the negative electrode active material and the capacity balance between the positive electrode and the negative electrode, the above-described control of the lower limit potential of the positive electrode is It is unnecessary. In other words, by designing the battery so that the positive electrode potential after the change of the negative electrode potential is 3.7 V or higher in a battery that reaches the end of discharge due to a change in the negative electrode potential, Based on the discharge termination control, it becomes unnecessary.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples.

[Test body 1]
On the surface of the aluminum foil, LiNi _1/3 Mn _1/3 Co _1/3 O ₂ (containing 0.8% by mass of tungsten) as a positive electrode active material, acetylene black as a conductive agent, and PVDF as a binder Were stacked in a mass ratio of 90: 5: 5 to produce a positive electrode. In addition, a negative electrode active material layer containing graphite as a negative electrode active material and PVDF as a binder at a mass ratio of 95: 5 was laminated on the surface of the copper foil to prepare a negative electrode. A power generation element was produced by winding the positive electrode and the negative electrode through a separator. After inserting this power generation element into the battery case, the battery lid was welded by laser welding. Through a liquid injection hole provided in the battery lid, an electrolyte solution in which LiPF ₆ was dissolved at 1 M in a solvent containing EC and EMC at a mass ratio of 30:70 was filled. A lithium ion secondary battery of the test body 1 having a battery cross section as shown in FIG. 1 and a battery capacity of 700 mAh was manufactured.

[Specimen 2]
Except that the positive electrode active material was LiNi _1/3 Mn _1/3 Co _1/3 O ₂ (containing 0.5% by mass of zirconium), the lithium ion of the test sample 2 was used using the same material as the test sample 1 described above. A secondary battery was manufactured.

[Evaluation]
About the lithium ion secondary battery of the test bodies 1 and 2, the charge lifetime of the battery was set to 4.15V, and the cycle life test which performs 500 charging / discharging cycles at 45 degreeC was implemented. In this cycle life test, the end-of-discharge voltage of each battery was set to 2.75V, 3.40V, 3.50V and 3.58V, respectively. The discharge end potential of the positive electrode (lower limit potential of the positive electrode) at each discharge end voltage is 2.95V, 3.60V, 3.70V, and 3.78V, respectively. The capacity retention rate of each battery corresponding to the discharge end potential of the positive electrode during discharge (the discharge capacity at the 500th cycle relative to the discharge capacity at the first cycle) was measured. The results are shown in Table 1.

  As shown in Table 1, in the test body 1 in which the positive electrode active material contains tungsten, when the discharge end potential is less than 3.7 V on the basis of lithium, the capacity retention rate is drastically reduced, and from the test body 2 that does not contain tungsten. It can also be seen that the drop is greatly reduced. On the contrary, in the test body 1, it can be seen that the capacity maintenance ratio can be made higher than that of the test body 2 by setting the discharge end potential to 3.7 V or more with respect to lithium. In the test sample 2, the capacity retention rate was hardly changed by the discharge end potential. It was found that when the positive electrode active material contains tungsten and the lower limit potential of the positive electrode with respect to the lithium reference is 3.7 V or more, the capacity retention rate in the cycle life is improved.

  As described above, the capacity maintenance rate in the charge / discharge cycle can be remarkably increased in the charge / discharge method and battery module of the lithium ion secondary battery of the present invention. Moreover, the lithium ion secondary battery of the present invention is excellent in output characteristics and capacity retention. Therefore, the present invention can be applied to electronic devices such as personal computers and communication terminals, automobiles, and the like.

DESCRIPTION OF SYMBOLS 1 Lithium ion secondary battery 2 Power generation element 3 Positive electrode 4 Negative electrode 5 Separator 6 Battery case 7 Battery cover 8 Injection plug 9 Negative electrode terminal

Claims (5)

A charge / discharge method for a lithium ion secondary battery comprising a positive electrode having a positive electrode active material,
The positive electrode active material contains tungsten;
A charging / discharging method for a lithium ion secondary battery, wherein a lower limit potential of the positive electrode is 3.7 V or more based on lithium.   The method for charging and discharging a lithium ion secondary battery according to claim 1, wherein the content of tungsten in the positive electrode active material is 0.05 mass% or more and 3 mass% or less. The positive electrode active material is a lithium-containing transition metal oxide having a layered structure,
The method for charging and discharging a lithium ion secondary battery according to claim 1, wherein the positive electrode active material contains nickel as a transition metal of the lithium-containing transition metal oxide. A lithium ion secondary battery comprising a positive electrode having a positive electrode active material;
A battery module comprising a mechanism for controlling charge and discharge of the lithium ion secondary battery,
The positive electrode active material contains tungsten;
The battery module, wherein the control mechanism controls the lower limit potential of the positive electrode to be 3.7 V or more with respect to lithium. A lithium ion secondary battery comprising a positive electrode having a positive electrode active material and a negative electrode having a negative electrode active material,
The positive electrode active material contains tungsten;
The negative electrode active material contains graphite,
The lithium ion secondary battery, wherein the minimum potential of the positive electrode is 3.7 V or more based on lithium.

Images