JP2017027928A - Method for manufacturing lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a lithium ion secondary battery, configured to reduce battery resistance even if a coating film including fluorine and phosphorus is formed on a surface of each positive electrode active material particle in an initial charging step.SOLUTION: A lithium ion secondary battery comprises: a positive electrode having a positive electrode active material layer including positive electrode active material particles; a negative electrode; and a nonaqueous electrolyte solution containing a fluorine-containing compound, provided that each positive electrode active material particle has a coating film including fluorine and phosphorus on its particle surface. In the lithium secondary battery, the positive electrode active material layer includes particles of one or both of metal phosphate and metal pyrophosphate. A method for manufacturing the lithium ion secondary battery, in initially charging the battery 1, includes: a first step SA1 of charging the battery to raise a voltage Vt of the battery to a first voltage in a lower decomposition area Ad; a second step SA2, SA2a of retaining the voltage Vt of the battery at the first voltage Vh; and a third step SA3 of charging the battery to a second voltage Ve higher than the first voltage Vh after the second step SA2, SA2a with a charging current larger than 1 C.SELECTED DRAWING: Figure 12

Description

  The present invention relates to a lithium ion secondary battery including a positive electrode plate having a positive electrode active material layer containing positive electrode active material particles, a negative electrode plate, and a non-aqueous electrolyte containing a compound containing fluorine.

  Conventionally, in a lithium ion secondary battery (hereinafter also simply referred to as a battery), the positive electrode potential is high, and therefore, the nonaqueous solvent of the nonaqueous electrolyte solution is likely to be oxidatively decomposed on the particle surface of the positive electrode active material particles. Are known. When hydrogen ions are generated by oxidative decomposition of the non-aqueous solvent, hydrogen ions may react with fluorine to generate hydrofluoric acid (HF) when the non-aqueous electrolyte has a compound containing fluorine. Then, due to the action of hydrofluoric acid, metal elements such as transition metals in the positive electrode active material particles are eluted, and the battery capacity is reduced. For this reason, in such a battery, there is a problem that the battery capacity is greatly reduced when a charge / discharge cycle test is performed.

To solve this problem, a technique is known in which metal phosphate particles such as lithium phosphate and metal pyrophosphate particles are included in the positive electrode active material layer. When the metal phosphate particles are included in the positive electrode active material layer, when the battery is charged for the first time, the above-described hydrofluoric acid reacts with the metal phosphate, so that fluorine and phosphorus are present on the surface of the positive electrode active material particles. Is formed. This coating prevents the non-aqueous electrolyte from coming into direct contact with the positive electrode active material. Therefore, after the coating is formed, the non-aqueous solvent is oxidized even if the positive electrode potential exceeds the oxidative decomposition potential of the non-aqueous solvent. It can suppress being decomposed. Therefore, it is possible to suppress a decrease in battery capacity after performing a charge / discharge cycle test on the battery.
For example, Patent Document 1 discloses a technique in which a positive electrode mixture layer (positive electrode active material layer) contains metal phosphate particles such as lithium phosphate and sodium phosphate.

JP 2014-103098 A

  However, it has been found that when the charging current is increased in the initial charging of the battery, the battery resistance tends to increase. The film containing fluorine and phosphorus is a resistor. However, when this coating film is formed, if the charging current is large, it is presumed that the oxidative decomposition of the nonaqueous electrolytic solution occurs excessively and the coating film is formed thick.

  The present invention has been made in view of the current situation, and in the step of charging the battery for the first time (initial charging step), while forming a film containing fluorine and phosphorus on the particle surface of the positive electrode active material particles, The present invention provides a method for producing a lithium ion secondary battery capable of reducing battery resistance.

  One embodiment of the present invention for solving the above problems includes a positive electrode having a positive electrode active material layer containing positive electrode active material particles, a negative electrode, and a non-aqueous electrolyte solution containing a fluorine-containing compound. The active material particle is a method of manufacturing a lithium ion secondary battery having a coating film containing fluorine and phosphorus on the particle surface, wherein the positive electrode active material layer is at least one of a metal phosphate and a metal pyrophosphate The step of charging the lithium ion secondary battery for the first time including particles includes charging the lithium ion secondary battery to increase the voltage of the lithium ion secondary battery to the first voltage in the lower decomposition zone. And a second step of maintaining the voltage of the lithium ion secondary battery at the first voltage, and after the second step, charging to a second voltage higher than the first voltage with a charging current greater than 1C. A third step of performing a method for producing a lithium ion secondary battery comprising a.

In this method of manufacturing a lithium ion secondary battery, in the initial charging step, after the first step, the battery voltage (inter-terminal voltage) is temporarily held at the first voltage in the lower decomposition region after the first step. One voltage constant voltage charging (hereinafter also referred to as CV charging) is performed, and then charging is performed to the second voltage in the third step.
For this reason, in the second step, oxidative decomposition of the non-aqueous electrolyte occurs while the battery voltage is maintained at the first voltage. However, the first voltage is set to a voltage within a low voltage range called a lower decomposition region even in a range where the nonaqueous electrolytic solution undergoes oxidative decomposition. For this reason, the oxidative decomposition of the non-aqueous electrolyte occurs only gradually, and a coating film containing fluorine and phosphorus can be formed thinly on the particle surface of the positive electrode active material particles, and the battery resistance can be kept low.

  In addition, after this coating is appropriately formed on the surface of each positive electrode active material particle, even if the voltage of the battery is within a range where the nonaqueous electrolyte undergoes oxidative decomposition, the nonaqueous electrolyte solution Oxidative decomposition is suppressed. This is thought to be because the generated coating prevents the non-aqueous electrolyte from coming into contact with the positive electrode active material particles.

The “lower decomposition zone” of the nonaqueous electrolytic solution refers to a voltage range from the lower limit of decomposition, which is the lower limit voltage at which the nonaqueous electrolytic solution is oxidatively decomposed, to a voltage 0.4 V higher than this. For example, when the decomposition lower limit voltage is 4.0V, the “lower decomposition zone” is 4.0 to 4.4V.
This is because when the voltage is kept within this range, the oxidative decomposition of the non-aqueous electrolyte does not become excessive.
Further, the “decomposition lower limit voltage” of the non-aqueous electrolyte is from the “decomposition lower limit potential (vs. Li / Li +)” of the non-aqueous electrolyte to the potential of the negative electrode (for example, 0.2 V (for a negative electrode using graphite particles). vs. Li / Li +)).
Further, the “lower decomposition potential (vs. Li / Li +)” of the non-aqueous electrolyte is a value detected by the following method. A measuring cell having a working electrode made of a Pt plate, a counter electrode made of metallic lithium, and a reference electrode, and using a nonaqueous electrolytic solution used for a battery as an electrolytic solution is prepared. Using an electrochemical measurement system (for example, manufactured by Solartron), the potential of the working electrode is in the range of 3.0 to 5.4 V (vs. Li / Li +) and the rate is 1 mV / sec. The CV measurement for raising and lowering is performed for two cycles. Further, the relationship between the positive electrode potential Ep (V (vs. Li / Li +)) and the current I (μA / cm ² ) flowing at that time when the potential of the working electrode is increased in the third cycle is acquired. From this relationship, the relationship (graph) between the positive electrode potential Ep (V (vs.Li/Li+)) and the differential value dI / dEp is obtained. An approximate straight line that overlaps a portion where the differential value dI / dEp rises linearly is drawn, and the value of the positive electrode potential Ep at which the approximate line becomes the differential value dI / dEp = 0 is set as the “decomposition lower limit potential of the non-aqueous electrolyte”. (vs.Li/Li+)"Epd (see FIGS. 6 and 7).

The composition of the metal phosphate particles contained in the positive electrode active material layer is, for example, an alkali metal phosphate represented by M ₃ PO ₄ (M: alkali metal) or M ₃ (PO ₄ ). ₂ A phosphate of a group 2 element represented by (M: a group 2 element) or a phosphate containing both alkali metal and group 2 metal. Furthermore, as the alkali metal phosphate, for example, lithium phosphate (Li ₃ PO ₄ ), sodium phosphate (Na ₃ PO ₄ ), potassium phosphate (K ₃ PO ₄ ), sodium dilithium phosphate (Li ₂ NaPO ₄ ). Examples of Group 2 element phosphates include magnesium phosphate (Mg ₃ (PO ₄ ) ₂ ) and calcium phosphate (Ca ₃ (PO ₄ ) ₂ ). Examples of the phosphate containing both alkali metal and Group 2 metal include sodium magnesium phosphate (MgNaPO ₄ ). In addition, as a metal phosphate, for example, a metal containing an element other than an alkali metal and a Group 2 element such as lithium aluminum germanium phosphate (LAGP: Li _1.5 Al _0.5 Ge _1.5 (PO ₄ ) ₃ ) Also included are phosphates.

Examples of the composition of the metal pyrophosphate particles include alkali metal pyrophosphate represented by M ₄ P ₂ O ₇ (M: alkali metal) and M ₂ P ₂ O ₇ (M: Group 2 element). And pyrophosphates of Group 2 elements represented by Further, examples of the alkali metal pyrophosphate include lithium pyrophosphate (Li ₄ P ₂ O ₇ ), sodium pyrophosphate (Na ₄ P ₂ O ₇ ), and potassium pyrophosphate (K ₄ P ₂ O ₇ ). . Examples of Group 2 element pyrophosphates include magnesium pyrophosphate (Mg ₂ P ₂ O ₇ ) and calcium pyrophosphate (Ca ₂ P ₂ O ₇ ).

Examples of the positive electrode active material forming the “positive electrode active material particles” include lithium transition metal composite oxides. Examples of the lithium transition metal composite oxide include lithium nickel cobalt manganese based composite oxide containing nickel (Ni), cobalt (Co) and manganese (Mn) as transition metals, and nickel and manganese as transition metals. Examples thereof include lithium nickel manganese composite oxide, lithium nickelate (LiNiO ₂ ), lithium cobaltate (LiCoO ₂ ), and lithium manganate (LiMn ₂ O ₄ ).

More specifically, a lithium nickel manganese composite oxide having a spinel crystal structure represented by the following general formula (1) can be used as the positive electrode active material.
_{Li (Ni x M y Mn 2} -xy) O 4 ··· (1)
However, x is x> 0, preferably 0.2 ≦ x ≦ 1.0.
Further, y is y ≧ 0, preferably 0 ≦ y <1.0.
Further, x + y <2.0.
“M” is any transition metal element other than Ni and Mn (eg, one or more selected from Fe, Co, Cu, Cr), or a typical metal element (eg, Zn, Al). 1 type or 2 types or more selected).
Note that whether or not the crystal structure of the positive electrode active material has a spinel structure can be determined by, for example, X-ray structure analysis (preferably single crystal X-ray structure analysis). Specifically, it can be determined by X-ray diffraction measurement using CuKα rays.

The “coating containing fluorine and phosphorus” may contain decomposition products of non-aqueous electrolyte components (electrolytes, non-aqueous solvents, additives, etc.) in addition to fluorine and phosphorus.
In the “positive electrode active material layer”, in addition to the positive electrode active material particles and at least one of metal phosphate and metal pyrophosphate, conductive materials such as graphite, carbon black, and acetylene black, polyvinylidene fluoride ( PVDF), polytetrafluoroethylene (PTFE), styrene butadiene rubber (SBR) and the like can be included.
Examples of the “negative electrode plate” include a negative electrode active material layer including negative electrode active material particles provided on a negative electrode current collector foil. Examples of the negative electrode active material particles include particles made of a carbon material capable of inserting and removing lithium such as graphite.

Examples of the non-aqueous solvent for the “non-aqueous electrolyte solution” include organic solvents such as dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, and the like. It can be used alone or in admixture of two or more. Fluorine-containing nonaqueous solvents such as fluoroethylene carbonate and 2,2,2-trifluoroethyl methyl carbonate can also be used.
As the electrolyte to be added to the "non-aqueous electrolyte" (supporting electrolyte), such as _{LiPF 6, LiBF 4, LiAsF 6} , LiSbF 6, LiCF 3 SO 3 as a supporting electrolyte containing fluorine. These Can be used alone or in combination of two or more.

In addition, the “nonaqueous electrolytic solution” may contain an additive other than the above electrolyte. Examples of the additive include fluoride and lithium bisoxalate borate (LiBOB). The fluoride, for _{example, AgF, CoF 2, CoF 3} , CuF, CuF 2, FeF 2, FeF 3, LiF, etc. _{MnF 2, MnF 3, SnF 2} , SnF 4, TiF 3, TiF 4, ZrF 4 is These may be used alone or in combination of two or more.
The “compound containing fluorine” contained in the nonaqueous electrolytic solution may be an electrolyte containing fluorine such as LiPF ₆ , an additive containing fluorine such as LiF, or a non-containing material containing fluorine such as fluoroethylene carbonate. An aqueous solvent may be used. Moreover, the compound containing the fluorine contained in a non-aqueous electrolyte may be only 1 type, and 2 or more types may be contained.
The charging current of 1C indicates the magnitude of the charging current that can charge the rated capacity of the battery in one hour.

As charging of the battery in the first step, charging by constant current charging (hereinafter also referred to as CC charging) or constant power charging can be employed. Alternatively, charging can be performed until the battery voltage reaches the first voltage by constant voltage charging (CV charging) in which the set voltage is equal to or higher than the first voltage.
The charging of the battery in the second step is constant voltage charging in which charging is performed while maintaining the first voltage.
Further, as charging of the battery in the third step, constant current charging in which the charging current is set to a value larger than 1C or charging by constant power charging in which the charging current is limited to a value larger than 1C can be adopted. . Alternatively, the battery voltage can be charged until the battery voltage becomes the second voltage by constant voltage charging in which the set voltage is higher than the second voltage.

  In the above-described method for manufacturing a lithium ion secondary battery, the second step is a process for producing a lithium ion secondary battery that maintains the voltage of the lithium ion secondary battery at the first voltage over a predetermined holding period. A manufacturing method is preferable.

  In this manufacturing method, the voltage of the battery is held at the first voltage over a predetermined holding period in the second step. For this reason, the coating film according to a holding | maintenance period can be reliably formed on the positive electrode active material particle surface.

  Furthermore, in the above-described method for manufacturing a lithium ion secondary battery, the holding period includes the battery resistance Rn of the battery manufactured by performing the second step without extending the holding period and then performing the third step. And instead of the holding period, the battery voltage of the extended holding battery manufactured by holding the first voltage over an extended holding period obtained by extending the holding period by a factor of 1.5 and then performing the third step. It is preferable to use a method for manufacturing a lithium ion secondary battery in which Rn = 0.98Re to 1.02Re when compared with Re.

The battery resistance after the initial charge decreases as the holding period is longer, that is, as the formation of a film containing fluorine and phosphorus on the surface of the positive electrode active material particles progresses. Even if the period is extended, the battery resistance does not change. In this case, the battery resistance Rn becomes Rn = 0.98Re to 1.02Re because the first voltage is held for the holding period (for example, 40 minutes), so that the positive electrode active material particle surface This shows that since a film containing fluorine and phosphorus is formed, even when the holding period is further extended by 1.5 times (for example, 60 minutes), there is almost no film formed. That is, if the first voltage is held for the holding period, the coating film is sufficiently formed so that the battery resistance Rn differs by no more than 2% as compared with the battery resistance Re of the extended holding battery.
Therefore, if the first voltage is held for such a holding period, the formation of the coating film containing fluorine and phosphorus can be immediately completed and the process can proceed to the third step that continues quickly. That is, a battery having a low battery resistance can be manufactured by appropriately forming a thin film capable of preventing oxidative decomposition of the nonaqueous solvent on the surface of the positive electrode active material particles.

  Or it is a manufacturing method of the above-mentioned lithium ion secondary battery, Comprising: The said holding period performs the said 2nd process, without extending the said holding period, Then, the battery resistance Rn of the battery manufactured by performing the said 3rd process And instead of the holding period, the battery voltage of the extended holding battery manufactured by holding the first voltage over an extended holding period obtained by extending the holding period by a factor of 1.5 and then performing the third step. A lithium ion secondary battery manufacturing method in which Rn = 0.99Re to 1.01Re when comparing with Re is preferable.

  In this manufacturing method, if the first voltage is held for the holding period, the coating film is sufficiently formed so that the battery resistance Rn differs by no more than 1% compared to the battery resistance Re of the extended holding battery. The Therefore, if the first voltage is held for such a holding period, the formation of the coating film containing fluorine and phosphorus can be immediately completed and the process can proceed to the third step that continues quickly. That is, it is possible to manufacture a battery in which a coating is further appropriately formed on the surface of the positive electrode active material particles.

  In the above-described method for manufacturing a lithium ion secondary battery, the second step includes the first voltage until the charge current of the lithium ion secondary battery is equal to or lower than a predetermined cutoff current value. The method of manufacturing a lithium ion secondary battery held in

In the second step, the battery voltage is maintained at the first voltage. However, due to variations in the battery, a difference occurs in the rate of film formation formed on the surface of the positive electrode active material particles. Therefore, when the period for maintaining the first voltage is the same, the film is formed on the surface of the positive electrode active material particles. The state such as the thickness of the coating is different, and a difference such as variation in battery resistance occurs. For this reason, in order to obtain a film with an appropriate thickness for any battery, it is necessary to set a longer holding period in accordance with a battery having a slower speed. For each battery, the holding period is longer. It may occur too much.
In contrast, in the above-described manufacturing method, in the second step, the first voltage is held until the charging current is equal to or lower than the cutoff current value, not the predetermined holding period. About each battery, the film of the same thickness can be formed in the positive electrode active material particle surface in the shortest time.

  Further, in the above-described method for manufacturing a lithium ion secondary battery, the cutoff current value is 2/5 the final current value at the end of the first step. And good.

  When the voltage of the battery is set to the first voltage in the first step (for example, by CC charging that is charged with a predetermined current) and then held at the first voltage in the second step, that is, when CV charging is performed, Initially, the curve rapidly decreases from the final current value at the end of the first step, then gradually decreases, and then gradually approaches 0 (a shape similar to the graph of y = 1−ex). Draw. At the beginning of the second step, since the battery voltage is set to the first voltage, the electrolytic solution undergoes oxidative decomposition one after another, and a large current flows as the decomposition current. However, with the passage of time, the metal phosphate contained in the positive electrode active material layer is consumed, a film is formed, and the oxidative decomposition of the electrolytic solution is suppressed, so that the charging current is gradually reduced.

  Based on this, as described above, if the cutoff current value in the second step is 2/5 of the final current value, most of the coating film containing fluorine and phosphorus formed on the particle surface of the positive electrode active material particles It can be formed in the second step, and a good film can be formed on the particle surface of the positive electrode active material particles while making the second step a very short time. Further, the battery resistance can be lowered (specifically, for example, about 7% smaller) than when the second step is not provided.

  In addition, in the above-described method for manufacturing a lithium ion secondary battery, the cutoff current value is 1/5 the final current value at the end of the first step. And good.

  As described above, when the cutoff current value in the second step is 1/5 of the final current value, most of the coating film containing fluorine and phosphorus formed on the surface of the positive electrode active material particles is formed in the second step. Thus, a good film can be formed on the surface of the positive electrode active material particles while making the second step a short time. Further, the battery resistance can be lowered (specifically, for example, about 10% smaller) than when the second step is not provided.

  Alternatively, in the method for manufacturing a lithium ion secondary battery described above, the final current value at the end of the first step is 1 C or more, and the cutoff current value is 0.05 C. A manufacturing method is preferable.

In the manufacturing method described above, the final current value in the first step is 1C or more, while the second step is performed until the cut-off current value is 0.05 C, which is sufficiently smaller than the final current value. Thus, if the second step is performed until the cut-off current value becomes 0.05 C, the cut-off current value is further reduced (for example, the cut-off current value is 0.02 C), The battery resistance is almost the same. That is, even if the cut-off current value is smaller than 0.05C, the time of the second step is extended, but the battery resistance cannot be expected to decrease. The formation of the coating is considered to be because almost the entire amount of metal phosphate (or metal pyrophosphate) contained in the positive electrode active material layer is consumed when the charging current reaches 0.05C. .
Thus, when the cut-off current value is 0.05 C, almost the entire amount of the film containing fluorine and phosphorus formed on the particle surface of the positive electrode active material particles can be formed in the second step in the shortest time. . Moreover, a good film can be formed, and the battery resistance can be lowered (specifically, about 15% smaller) than when the second step is not provided.

  Furthermore, in the method for producing a lithium ion secondary battery according to any one of the above, at least one of the metal phosphate and the metal pyrophosphate contained in the positive electrode active material layer has an average particle diameter Is a method for manufacturing a lithium ion secondary battery having particles of 1.5 μm or less.

  In this manufacturing method, the particle diameter of particles such as metal phosphate such as lithium phosphate contained in the positive electrode active material layer is 1.5 μm or less in average particle diameter. For this reason, if the addition amount is the same, the number of particles and the total amount of surface area increase, so that the reaction with the generated hydrogen fluoride (hydrofluoric acid) is likely to occur, the coating can be formed in a short time, and the time required for the second step As a result, it can contribute to shortening the time required for the initial charging process.

  Furthermore, in the method for producing a lithium ion secondary battery according to any one of the above, the potential of the positive electrode is at least partly within an operating range (SOC = 0 to 100%) of the lithium ion secondary battery. A method for producing a lithium ion secondary battery having a voltage of 4.5 V (vs. Li / Li +) or higher is preferable.

  In the lithium ion secondary battery according to this manufacturing method, the potential of the positive electrode is at least 4.5 V (vs. Li / Li +) in at least a part of the range of SOC = 0 to 100%. For this reason, the non-aqueous electrolyte solution (non-aqueous solvent) is easily oxidized and decomposed on the surface of the positive electrode active material particles to generate hydrogen ions. Moreover, the non-aqueous electrolyte contains a compound containing fluorine as described above. For this reason, hydrofluoric acid is easily generated from hydrogen ions and fluorine. However, in this battery manufacturing method, as described above, in the initial charging step (second step), since the coating film containing fluorine and phosphorus is formed on the particle surface of the positive electrode active material particles, the initial charging step is performed. Later, oxidative decomposition of the nonaqueous electrolytic solution (nonaqueous solvent) can be suppressed.

  Furthermore, in the method for manufacturing a lithium ion secondary battery according to any one of the foregoing, the first step and the third step are lithium ion secondary batteries that are charged with a constant current at a predetermined current value of 3C or more. A battery manufacturing method is preferable.

  In this manufacturing method, the first step and the third step are CC charged with a current value of 3C or more. Thereby, the time required for the first step can be shortened, and the step of charging the battery for the first time (initial charging step) can be further shortened.

It is a perspective view of the lithium ion secondary battery which concerns on Embodiment 1, 2 and a modification. It is the longitudinal cross-sectional view which cut | disconnected the lithium ion secondary battery which concerns on Embodiment 1, 2 and a modification with the plane in alignment with a battery horizontal direction and a battery vertical direction. It is an expanded view of the electrode body which concerns on Embodiment 1, 2 and a modification, and shows the state which piled up the positive electrode plate and the negative electrode plate through the separator. It is explanatory drawing which concerns on Embodiment 1, 2 and a modification, and shows typically the mode of the particle | grain surface vicinity among the cross sections of a positive electrode active material particle. 4 is a flowchart illustrating a procedure of each process included in the initial charging process according to the first embodiment. It is a graph which shows the relationship between the positive electrode electric potential Ep measured using the measurement cell about the non-aqueous electrolyte used for the battery which concerns on Embodiment 1, 2 and a modification, and the electric current I flowing at that time. It is a graph which shows the relationship between the positive electrode electric potential Ep and the differential value dI / dEp obtained from the graph shown in FIG. It is a graph which shows the relationship between the 1st voltage and battery resistance ratio in each battery which concerns on Example 1, 2 and Comparative example 1,2. It is a graph which shows the thickness of the film produced | generated by the positive electrode active material particle of each battery which concerns on Examples 1, 2 and Comparative Examples 1-3. It is a graph which shows the relationship between a holding | maintenance period and battery resistance ratio about each battery which concerns on Examples 4-13 and Comparative Examples 4-7. It is a graph which shows the relationship between the average particle diameter of a metal phosphate, and the holding | maintenance period from which battery resistance ratio will be 1.00. It is a flowchart which shows the procedure of each process which concerns on Embodiment 2 and a modification, and is included in an initial charge process. 10 is a flowchart illustrating a procedure of a second step in the initial charging step according to the second embodiment. FIG. 10 is a graph showing a relationship between a charging time t, a battery terminal voltage Vt, and a charging current Ib in the initial charging step according to the second embodiment and a modified embodiment. It is a graph which shows the relationship between the cut-off electric current value Ibc and battery resistance ratio of a 2nd process among initial charge processes according to Embodiment 2 and a modification. It is a flowchart which shows the procedure of a 2nd process among initial stage charge processes in connection with a deformation | transformation form.

Hereinafter, a first embodiment of the present invention will be described with reference to the drawings. 1 and 2 show a lithium ion secondary battery (hereinafter also simply referred to as “battery”) 1 according to the present embodiment. FIG. 3 shows a development view of the electrode body 20 constituting the battery 1. In the following description, the battery thickness direction BH, the battery lateral direction CH, and the battery vertical direction DH of the battery 1 are defined as the directions shown in FIGS. 1 and 2.
The battery 1 is a rectangular and sealed lithium ion secondary battery mounted on a vehicle such as a hybrid vehicle or an electric vehicle. The battery 1 includes a battery case 10, an electrode body 20 and a nonaqueous electrolyte solution 40 accommodated therein, a positive terminal 50 and a negative terminal 51 supported by the battery case 10, and the like. The battery 1 is a battery that operates between terminal voltages Vt = 3.5 to 4.9 V (SOC = 0 to 100%) between the positive electrode terminal 50 and the negative electrode terminal 51. Further, in the range of SOC = 0 to 100%, the positive electrode potential Ep fluctuates within a range of Ep = 3.7 to 5.0 V (vs. Li / Li +), and the negative electrode potential En is En = 0.2. ~ 0.1V (vs. Li / Li +).

  Among these, the battery case 10 has a rectangular parallelepiped shape and is made of metal (in this embodiment, aluminum). This battery case 10 includes a rectangular parallelepiped box-shaped case main body member 11 whose upper side is opened, and a rectangular plate-shaped case lid member 13 welded in a form to close the opening 11h of the case main body member 11. . The case lid member 13 is provided with a safety valve 14 that opens when the internal pressure of the battery case 10 reaches a predetermined pressure. The case lid member 13 is formed with a liquid injection hole 13 h that communicates the inside and outside of the battery case 10 and is hermetically sealed with a sealing member 15.

  Further, the case lid member 13 has a positive terminal 50 and a negative terminal 51 formed of an internal terminal member 53, an external terminal member 54, and a bolt 55, respectively, via an internal insulating member 57 and an external insulating member 58 made of resin. It is fixed. The positive terminal 50 is made of aluminum, and the negative terminal 51 is made of copper. In the battery case 10, the positive electrode terminal 50 is connected and connected to the positive electrode current collector 21 m of the positive electrode plate 21 in the electrode body 20 described later. Further, the negative electrode terminal 51 is connected to the negative electrode current collector 31 m of the negative electrode plate 31 in the electrode body 20 and is conductive.

  Next, the electrode body 20 will be described (see FIGS. 2 and 3). The electrode body 20 has a flat shape and is accommodated in the battery case 10. The electrode body 20 is obtained by winding a belt-like positive electrode plate 21 and a belt-like negative electrode plate 31 on each other via a pair of belt-like separators 39 and compressing them in a flat shape.

The positive electrode plate 21 is formed by providing a positive electrode active material layer 23 in a band shape on a region extending in a part of the width direction and extending in the longitudinal direction among both main surfaces of a positive electrode current collector foil 22 made of a band-shaped aluminum foil. The positive electrode active material layer 23 includes positive electrode active material particles 24, a conductive material (conductive aid) 26, a binder 27, and lithium phosphate particles (metal phosphate particles) 28, which will be described later. In the present embodiment, acetylene black (AB) is used as the conductive material 26, polyvinylidene fluoride (PVDF) is used as the binder 27, and lithium phosphate (Li ₃ PO ₄ ) particles (powder) are used as the metal phosphate particles 28. Used.

  The mixing ratio of the positive electrode active material particles 24, the conductive material 26, and the binder 27 is 89: 8: 3 by weight. The compounding ratio of the metal phosphate particles 28 is 3 parts by weight, based on the positive electrode active material particles 24 (100 parts by weight). Also, one end of the positive electrode current collector foil 22 in the width direction is a positive electrode current collector part 21 m where the positive electrode current collector foil 22 is exposed without the positive electrode active material layer 23 in the thickness direction of the positive electrode current collector foil 22. Yes. The positive electrode terminal 50 described above is welded to the positive electrode current collector 21m.

In the present embodiment, the positive electrode active material particles 24 are made of lithium transition metal composite oxide, specifically, LiNi _0.5 Mn _1.5 O ₄ which is one of lithium nickel manganese composite oxide having a spinel crystal structure. Particles. A coating 25 containing fluorine and phosphorus is formed on the particle surface 24n of the positive electrode active material particles 24 (see FIG. 4). In addition to fluorine and phosphorus, the coating 25 includes decomposition products of other components (electrolyte and nonaqueous solvent) of the nonaqueous electrolytic solution 40.

  Next, the negative electrode plate 31 will be described. This negative electrode plate 31 is provided with a negative electrode active material layer 33 in a band shape on a region extending in a part of the width direction and extending in the longitudinal direction out of both main surfaces of a negative electrode current collector foil 32 made of a strip-shaped copper foil. . The negative electrode active material layer 33 includes negative electrode active material particles, a binder, and a thickener. In this embodiment, graphite particles are used as negative electrode active material particles, styrene butadiene rubber (SBR) is used as a binder, and carboxymethyl cellulose (CMC) is used as a thickener. Also, one end in the width direction of the negative electrode current collector foil 32 is the negative electrode current collector part 31m where the negative electrode active material layer 33 is not present in the thickness direction of the negative electrode current collector foil 32 and the negative electrode current collector foil 32 is exposed. Yes. The negative electrode terminal 51 described above is welded to the negative electrode current collector 31m. The separator 39 is a porous film made of resin and has a strip shape.

Next, the nonaqueous electrolytic solution 40 will be described. The non-aqueous electrolyte 40 is accommodated in the battery case 10, a part of the non-aqueous electrolyte 40 is impregnated in the electrode body 20, and the rest is accumulated at the bottom of the battery case 10 as an excess liquid. The electrolyte of this nonaqueous electrolytic solution 40 is lithium hexafluorophosphate (LiPF ₆ ), and its concentration is 1.0M. The nonaqueous solvent of the nonaqueous electrolytic solution 40 is a mixed organic solvent in which fluoroethylene carbonate (FEC) and 2,2,2-trifluoroethyl methyl carbonate are mixed at a volume ratio of 1: 1. As described above, this nonaqueous electrolytic solution 40 includes, as the compound 41 containing fluorine, LiPF ₆ that is a supporting electrolyte, fluoroethylene carbonate (FEC) that is a nonaqueous solvent, and 2,2,2-trifluoroethyl. Has methyl carbonate.

Next, a method for manufacturing the battery 1 will be described. First, the positive electrode plate 21 is formed. Specifically, positive electrode active material particles 24 made of LiNi _0.5 Mn _1.5 O ₄ which is a lithium nickel manganese composite oxide having a spinel structure are prepared. The positive electrode active material particles 24, the conductive material 26 (acetylene black), the binder 27 (polyvinylidene fluoride), and the metal phosphate particles 28 (lithium phosphate particles, average particle diameter D50 = 3.0 μm). ) With a solvent (NMP in this embodiment) to produce a positive electrode paste. The mixing ratio of the positive electrode active material particles 24, the conductive material 26, and the binder 27 is 89: 8: 3 by weight as described above, and the positive electrode active material particles 24 are 100 parts by weight. When this is done, the metal phosphate particles 28 are added in a proportion of parts by weight. As will be described later, when the metal phosphate particles 28 having an average particle diameter D50 = 1.5 μm or 0.8 μm are used, the particle diameter is adjusted to a desired size using a wet bead mill. .

Thereafter, the positive electrode paste is applied to one main surface of the positive electrode current collector foil 22 made of a strip-shaped aluminum foil and dried to form the positive electrode active material layer 23. Further, the positive electrode paste is applied to the other main surface of the positive electrode current collector foil 22 and dried to form the positive electrode active material layer 23. Then, this is pressed and the positive electrode plate 21 is obtained.
Separately, the negative electrode plate 31 is formed by a known method.

Next, the positive electrode plate 21 and the negative electrode plate 31 are overlapped with each other via a pair of separators 39 and wound using a winding core. Further, the electrode body 20 is formed by compressing it into a flat shape.
Separately, a case lid member 13, an internal terminal member 53, an external terminal member 54, a bolt 55, an internal insulating member 57 and an external insulating member 58 are prepared. Then, the positive terminal 50 and the negative terminal 51 including the internal terminal member 53, the external terminal member 54, and the bolt 55 are fixed to the case lid member 13 via the internal insulating member 57 and the external insulating member 58, respectively. Thereafter, the positive electrode terminal 50 and the negative electrode terminal 51 integrated with the case lid member 13 are welded to the positive electrode current collector 21m and the negative electrode current collector 31m of the electrode body 20, respectively.
Next, after housing the electrode body 20 in the case body member 11, the case cover member 13 is welded to the opening portion of the case body member 11 to form the battery case 10.

Separately, a non-aqueous electrolyte 40 is prepared. Specifically, LiPF ₆ is dissolved in a mixed organic solvent in which fluoroethylene carbonate and 2,2,2-trifluoroethyl methyl carbonate are mixed at a volume ratio of 1: 1 so as to have a concentration of 1.0M. Then, the nonaqueous electrolytic solution 40 is injected into the battery case 10 through the injection hole 13h, and the electrode body 20 is impregnated with the nonaqueous electrolytic solution 40. Thereafter, the liquid injection hole 13 h is temporarily sealed to obtain a battery 1.

Next, the battery 1 is initially charged (initial charging step). In this initial charging step, a coating 25 containing fluorine and phosphorus is formed on the particle surface 24n of the positive electrode active material particles 24 together with the initial charging.
Specifically, as the first charging step, first, the battery 1 is connected to a CC-CV charging / discharging device (not shown), and as shown in FIG. The voltage Vt is increased to Vt = 4.1V (first voltage Vh) (first step S1). Next, switching to CV charging with a terminal voltage Vt = 4.1V is performed. That is, the inter-terminal voltage Vt is held at the first voltage Vh = 4.1 V over the holding period Tk = 60 minutes (second step S2). After that, constant current charging (CC charging) is performed until the inter-terminal voltage Vt reaches the second voltage Ve at a constant current of 3.0 C, specifically, until Vt = Ve = 4.9 V (third charging). Step S3).

  Specifically, at the time of the above-described initial charging, the inter-terminal voltage Vt is set to the first voltage Vh = 4.1 V (positive electrode potential Ep of the positive electrode plate 21 = 4.3 V (vs. Li / Li +)) mainly in the second step. While the negative electrode potential En of the negative electrode plate 31 is maintained at 0.2 V (vs. Li / Li +), a coating 25 containing fluorine and phosphorus is formed on the particle surface 24 n of the positive electrode active material particles 24. . At this time, the positive electrode potential Ep = 4.3V (vs. Li / Li +) is higher by 0.1 V than the decomposition lower limit potential Epd = 4.2 V (vs. Li / Li +), as will be described later. It is a value. In addition, the first voltage Vh = Vt = 4.1V, which is the inter-terminal voltage held, is 0.1V higher than the decomposition lower limit voltage Vtd = 4.0V.

Although the mechanism by which the film 25 is formed is not clear, the following is conceivable. That is, when the positive electrode potential (oxidation-reduction potential) Ep of the positive electrode plate 21 (positive electrode active material particles 24) is equal to or higher than the decomposition lower limit potential Epd described later, the particle surface 24n of the positive electrode active material particles 24 is in contact with the surface 24n. The nonaqueous solvent of the nonaqueous electrolyte solution 40 (in this embodiment, fluoroethylene carbonate (FEC) and 2,2,2-trifluoroethylmethyl carbonate) is oxidized and decomposed to generate hydrogen ions. This hydrogen ion is a compound 41 containing fluorine in the nonaqueous electrolytic solution 40 (in this embodiment, LiPF _{6 as} a supporting electrolyte, fluoroethylene carbonate (FEC) as a solvent, 2,2,2-trifluoroethyl methyl carbonate). Reacts with hydrofluoric acid in the interior to produce hydrofluoric acid (HF). This hydrofluoric acid reacts with the metal phosphate (lithium phosphate) particles 28 contained in the positive electrode active material layer 23 to form a film 25 containing fluorine and phosphorus on the particle surface 24 n of the positive electrode active material particles 24. It is thought.
Thereafter, the temporary sealing of the battery is released, and the main sealing is performed under reduced pressure. Furthermore, various inspections are performed. Thus, the battery 1 is completed.

(Measurement of decomposition lower limit voltage and decomposition lower limit potential)
Next, in the battery 1 having the above-described configuration, among the positive electrode potential Ep of the positive electrode plate 21 (positive electrode active material particles 24), the lowest positive electrode potential Ep at which oxidative decomposition of the nonaqueous electrolyte solution (nonaqueous solvent) 40 occurs. The decomposition lower limit potential Epd is detected as follows. First, a measurement cell using a nonaqueous electrolytic solution 40 used for the battery 1 is prepared, which has a working electrode made of a Pt plate, a counter electrode made of metallic lithium, and a reference electrode. Using the electrochemical measurement system manufactured by Solartron, the potential of the working electrode was increased at a rate of 1 mV / sec over the range of 3.0 to 5.4 V (vs. Li / Li +) for this measurement cell. The descending CV measurement is performed for two cycles. Further, the relationship between the positive electrode potential Ep (V (vs.Li/Li+)) and the current I (μA / cm ² ) flowing at that time when the potential of the working electrode is increased in the third cycle is obtained (FIG. 6). Note that the current on the charging side is a positive value. From this relationship, a relationship between the positive electrode potential Ep (V (vs.Li/Li+)) and the differential value dI / dEp is obtained (FIG. 7). In a portion where the differential value dI / dEp rises linearly with the rise of the positive electrode potential Ep, an approximate straight line L overlapping this change is drawn, and this approximate straight line L shows the value of the positive electrode potential Ep at which the differential value dI / dEp = 0. The “lower decomposition potential (vs. Li / Li +)” Epd of the non-aqueous electrolyte 40 is used.

Measurement was performed on the non-aqueous electrolyte 40 (fluoroethylene carbonate (FEC) + 2,2,2-trifluoroethylmethyl carbonate (1: 1) and LiPF ₆ : 1.0M) used in the battery 1 as described above. FIG. 6 shows the relationship between the positive electrode potential Ep (V (vs.Li/Li+)) and the current I (μA / cm ² ) flowing at that time, and the positive electrode potential Ep (V (vs.Li/Li+)) and the differential The relationship with the value dI / dEp is shown in FIG.

  According to the graph of the positive electrode potential Ep versus the current I shown in FIG. 6, in the range of the positive electrode potential Ep = 3.3 to 4.1 V (vs. Li / Li +), the current I linearly increases as the positive electrode potential Ep increases. Seems to increase. However, in the range of the positive electrode potential Ep = 4.2 V (vs. Li / Li +) or more, it seems that the current I increases at an accelerated rate as the positive electrode potential Ep increases.

  Therefore, a differential value dI / dEp was calculated to obtain a graph of positive electrode potential Ep versus differential value dI / dEp (see FIG. 7). Then, in the range of the positive electrode potential Ep = 4.4 to 5.0 V (vs. Li / Li +), the differential value dI / dEp increases linearly with the increase of the positive electrode potential Ep (that is, in an accelerated manner). (In a quadratic function, the current I increases in proportion to the square of the potential). Therefore, an approximate straight line L that fits the range in which the differential value dI / dEp increases linearly is drawn. The value of the positive electrode potential Ep (that is, the X intercept in the graph of FIG. 7) at which the approximate line L becomes the differential value dI / dEp = 0 is Ep = 4.2 V (vs. Li / Li +). Therefore, the positive electrode potential Ep = 4.2 V (vs. Li / Li +) is set as the decomposition lower limit potential Epd of the nonaqueous electrolytic solution 40 according to the present embodiment. This is because the oxidative decomposition of the non-aqueous solvent is considered to increase in an accelerated manner (second-order function) as the positive electrode potential Ep increases when the positive electrode potential Ep exceeds the decomposition lower limit potential Epd.

  As described above, the battery 1 uses graphite as the negative electrode active material, and the negative electrode potential En is constant at En = 0.2 V (vs. Li / Li +). Therefore, the voltage Vt between the terminals of the battery 1 when the positive electrode plate 21 is at the decomposition lower limit potential Epd (= 4.2 V (vs. Li / Li +)) is Vt = Ep−En = 4.2−. 0.2 = 4.0V. Therefore, this value is set as the “decomposition lower limit voltage” Vtd (= 4.0 V) in the battery 1.

  Further, in the battery 1, the “lower decomposition zone” Ad of the non-aqueous electrolyte is defined as a range of Ad = Vtd to Vtd + 0.4 using the decomposition lower limit voltage Vtd. Specifically, the lower decomposition zone Ad is in the range of 4.0 to 4.4 V (see FIG. 8).

(Examples 1 and 2 and Comparative Examples 1 to 3)
Next, tests performed to verify the effects of the present invention and the results thereof will be described. As shown in Table 1 below, batteries using the same positive electrode plate 21, negative electrode plate 31, separator 39 and non-aqueous electrolyte solution 40 as those of the battery 1 were prepared. The test was conducted under various test conditions. Therefore, in the positive electrode active material layer 23 of the battery of each example, when the positive electrode active material particles 24 included in the positive electrode active material layer 23 are 100 parts by weight, lithium phosphate particles having an average particle diameter D50 = 3.0 μm. 3 parts by weight of (LPO) is contained (see Table 1).

  In the battery of Comparative Example 1, the inter-terminal voltage Vt was increased to the inter-terminal voltage Vt = 3.8 V, which is lower than the lower decomposition area Ad = 4.0 to 4.4 V in the initial charging (first step). Thereafter, this inter-terminal voltage Vt (first voltage Vh) = 3.8 V was held for the holding period Tk = 60 minutes (second step). Thereafter, constant current charging at a CC charging rate of 3.0 C was performed until the inter-terminal voltage Vt reached the second voltage Ve = 4.9 V (third step), and the initial charging was completed. The total charging time required for the initial charging of the battery of Comparative Example 1 is 80 minutes.

  In the battery of Example 1, in the initial charge, the inter-terminal voltage Vt in the lower decomposition zone Ad is increased to the inter-terminal voltage Vt = 4.1 V (first step), and then the inter-terminal voltage Vt ( The first voltage Vh) = 4.1 V was held for the holding period Tk = 60 minutes (second step). Thereafter, constant current charging at a CC charging rate of 3.0 C was performed until the inter-terminal voltage Vt reached the second voltage Ve = 4.9 V (third step), and the initial charging was completed. The total charging time required for the initial charging is 80 minutes.

  The second embodiment is different from the first embodiment only in the inter-terminal voltage Vt to be held. That is, the inter-terminal voltage Vt is increased to Vt = 4.4 V in the lower decomposition zone Ad, and this inter-terminal voltage Vt (first voltage Vh) = 4.4 V is held for the holding period Tk = 60 minutes, Thereafter, CC charging was performed at a CC charging rate of 3.0C. The total charging time required for the initial charging is 80 minutes.

  Also in Comparative Example 2, only the inter-terminal voltage Vt to be held was different from that in Example 1. However, the inter-terminal voltage Vt is increased to Vt = 4.7 V, which exceeds the lower decomposition zone Ad, and this inter-terminal voltage Vt (first voltage Vh) = 4.7 V is held for the holding period Tk = 60 minutes, Thereafter, CC charging was performed at a CC charging rate of 3.0C. The total charging time required for the initial charging is 80 minutes.

  Unlike the comparative examples 1 and 2 and the examples 1 and 2, the comparative example 3 does not include the second step of holding the voltage. That is, from the beginning of the initial charging, constant current charging at a CC charging rate of 3.0 C was performed until the inter-terminal voltage Vt reached the second voltage Ve = 4.9 V, and the initial charging was completed. The total charging time for the initial charging is 20 minutes shorter than the others.

  For the batteries of Comparative Examples 1 to 3 and Examples 1 and 2, the battery resistance (IV resistance) was measured after the completion of the initial charge. Specifically, in a temperature environment of 25 ° C., each battery was adjusted to 60% SOC, discharged at a constant current of 0.3 C for 10 seconds, and voltage change before and after the discharge was measured. Furthermore, only the discharge current value was increased in the order of 1C, 3C, and 5C, while other than that, discharging was performed under the same conditions as described above, and voltage changes before and after discharging for 10 seconds were measured. After that, these data are plotted on a coordinate plane with the horizontal axis representing the discharge current value and the vertical axis representing the voltage change before and after the discharge, and an approximate straight line (primary expression) is calculated by the least square method, and the slope is expressed as IV resistance. Obtained as value. Then, the “battery resistance ratio” of other batteries was calculated using the battery resistance (IV resistance) of the battery of Example 2 as a reference (= 1.00). The results are shown in Table 1 and FIG. FIG. 8 is a graph showing the relationship between the first voltage Vh and the battery resistance ratio in each battery. However, the results of Comparative Example 3 are not shown in FIG.

  Further, the batteries of Comparative Examples 1 to 3 and Examples 1 and 2 were disassembled to take out the positive electrode active material particles 24, and formed on the particle surface 24n of the positive electrode active material particles 24 using a TEM (transmission electron microscope). The thickness of the coating film 25 containing fluorine and phosphorus was measured (n = 3). The results are shown in Table 1 and FIG. FIG. 9 is a graph showing the thickness of the coating formed on the positive electrode active material particles of each battery.

  From Table 1 and FIG. 8, it can be seen that when the holding period Tk is Tk = 60 minutes, the battery resistance ratio is the lowest when the first voltage Vh is Vh = 4.1 V (Example 1). . Further, even when the first voltage Vh is Vh = 4.4 V (Example 2), it can be seen that the battery resistance ratio is 1.05, and the resistance only increases by about 5%. In Examples 1 and 2, the first voltage Vh in the second step is set to Vh = 4.1V, which is slightly larger than the above-described decomposition lower limit voltage Vtd = 4.0V, or Vh = 4.4V, which is slightly larger. Therefore, oxidative decomposition of the nonaqueous electrolytic solution 40 occurs during the second step. However, the first voltage Vh is set to a voltage within a low voltage range of the lower decomposition zone Ad (= 4.0 to 4.4 V) even in a range where the nonaqueous electrolytic solution undergoes oxidative decomposition. For this reason, the oxidative decomposition of the non-aqueous electrolyte 40 occurs only gradually, preventing the formation of the coating 25 containing too thick fluorine and phosphorus on the particle surface 24n of the positive electrode active material particles 24, thereby reducing the battery resistance. It is thought that it was possible to keep it low. This is because the thickness tt of the coating 25 shown in Table 1 and FIG. 9 is tt = 10 nm in Example 1 and tt = 13 nm in Example 2, which is smaller than those in Comparative Examples 1 to 3. It is supported.

  On the other hand, in Comparative Example 2, the first voltage Vh in the second step was set to Vh = 4.7V, which is much higher than the decomposition lower limit voltage Vtd = 4.0V (exceeding the lower decomposition area Ad). Therefore, during the second step, a large current flows and a large amount of oxidative decomposition of the non-aqueous electrolyte solution 40 occurs at a time, and a coating 25 containing too thick fluorine and phosphorus is formed on the particle surface 24n of the positive electrode active material particles 24. As a result, the battery resistance is considered to be higher than in Examples 1 and 2. This point is supported by the fact that the thickness tt of the coating film 25 shown in Table 1 and FIG. 9 is tt = 18 nm in Comparative Example 2, which is thicker than those in Examples 1 and 2.

  In Comparative Example 3, the second step was not provided, and CC charging was performed with a large current at a charge rate of 3.0 C from the beginning, so that a large amount of oxidative decomposition of the nonaqueous electrolytic solution 40 occurred at a time. 25 is formed. This point is supported by the fact that the thickness tt of the coating film 25 shown in Table 1 and FIG. 9 is the thickest at tt = 20 nm in Comparative Example 3.

  In Comparative Example 1, the first voltage Vh in the second step was set to Vh = 3.8 V lower than the decomposition lower limit voltage Vtd = 4.0 V (below the lower decomposition area Ad). For this reason, it is considered that almost no oxidative decomposition of the nonaqueous electrolytic solution 40 occurred during the second step, and almost no film was formed in the second step. However, it is considered that the thick film 25 was formed by CC charging at a large current of a charging rate of 3.0 C in the third process thereafter. Therefore, the comparative example 1 has a generation pattern similar to that of the comparative example 3 in view of the generation of the film. This point is supported by the fact that the thickness tt of the coating 25 shown in Table 1 and FIG. 9 is tt = 18 nm in Comparative Example 1 and is thicker than those in Examples 1 and 2. Further, it is confirmed that the battery resistance is higher than those in Examples 1 and 2.

  From the above, the first voltage Vh in the second step is set to the lower decomposition lower limit voltage Vtd = 4.0V, and the upper decomposition lower limit voltage Vtd + 0.4V (= 4.4V), that is, the lower decomposition region described above. It can be understood that by setting the voltage within Ad (Vt = 4.0 to 4.4 V), the thin film 25 can be formed and the battery resistance can be lowered.

(Comparative Examples 3-6, Examples 1, 3-13)
Next, the holding periods Tk were varied for the batteries of Comparative Examples 3-6 and Examples 1, 3-13, in which the average particle diameter of the metal phosphate (LPO) added to the positive electrode active material layer was varied. A test was conducted to determine the battery resistance ratio (see Table 2). Specifically, a battery in which 3.00% by mass of lithium phosphate particles (LPO) having an average particle diameter D50 = 3.0 μm is added to the positive electrode active material layer 23 as in Example 1 described above is prepared, and the initial charge is performed. The battery is charged until the inter-terminal voltage Vt of the battery reaches 4.1 V (first step). Thereafter, the first voltage Vh is set to 4.1 V, and the holding period Tk is set to 0 minutes, 20 minutes, 40 minutes, 60 minutes, and 90 minutes (second step), and then the constant current charging at the CC charge rate of 3.0 C is performed. This was performed until the inter-terminal voltage Vt reached the second voltage Ve = 4.9 V (third step). These are referred to as Comparative Example 3 and Examples 1, 3 to 5. The example of the holding period Tk = 0 minutes corresponds to the above-described comparative example 3, and the example of the holding period Tk = 60 minutes corresponds to the above-described first embodiment.

  Further, unlike Example 1 and the like, lithium phosphate particles (LPO) having an average particle diameter D50 = 1.5 μm are formed on the positive electrode active material layer 23, and 3 wt% based on the positive electrode active material particles 24 (100 parts by weight). A part-added battery is prepared, and charged in the initial charge until the terminal voltage Vt of the battery reaches Vt = 4.1 V (first step). Thereafter, the inter-terminal voltage Vt is set to the first voltage Vh = 4.1 V, and the holding period Tk is set to 0 minutes, 10 minutes, 20 minutes, 30 minutes, and 60 minutes (second step), and then the CC charge rate is 3.0 C. The constant current charging was performed until the inter-terminal voltage Vt reached the second voltage Ve = 4.9 V (third step). These are referred to as Comparative Example 4 and Examples 6 to 9.

  Further, unlike Example 1 and the like, the cathode active material layer 23 is made of lithium phosphate particles (LPO) having an average particle diameter D50 = 0.8 μm, and 3 wt.% Based on the cathode active material particles 24 (100 parts by weight). A part-added battery is prepared, and charged in the initial charge until the terminal voltage Vt of the battery reaches Vt = 4.1 V (first step). Thereafter, the inter-terminal voltage Vt is set to the first voltage Vh = 4.1 V, and the holding period Tk is set to 0 minutes, 10 minutes, 20 minutes, 30 minutes, and 60 minutes (second step), and then the CC charge rate is 3.0 C. The constant current charging was performed until the inter-terminal voltage Vt reached the second voltage Ve = 4.9 V (third step). These are referred to as Comparative Example 5 and Examples 10 to 13.

  As Comparative Example 6, the same battery as in Example 1 was subjected to 180 minutes until the voltage Vt between the terminals reached the second voltage Ve = 4.9 V at the CC charge rate of 0.33 C from the beginning as the initial charge. Constant current charging was performed. Since the battery of Comparative Example 6 is charged at a low charge rate (charge current) of 0.33 C, even if oxidative decomposition of the non-aqueous electrolyte 40 (non-aqueous solvent) occurs during the initial charge. A large amount of decomposition does not occur at one time. Therefore, it is considered that the coating 25 containing fluorine and phosphorus is gradually formed on the particle surface 24n of the positive electrode active material particles 24, and it is considered that the coating 25 is thin, and the battery resistance (IV resistance) is low.

  For the batteries of Comparative Examples 3-6 and Examples 1, 3-13, the battery resistance (IV resistance) was measured by the same method as the battery of Example 1 and the like after the completion of the initial charge. Then, the “battery resistance ratio” of other batteries was calculated using the battery resistance (IV resistance) of the battery of Comparative Example 6 as a reference (= 1.00). The results are shown in Table 2 and FIG. FIG. 10 is a graph showing the relationship between the holding period Tk and the battery resistance ratio in each battery. However, the results of Comparative Example 6 are not shown in FIG.

  In Table 2 and FIG. 10, first, the batteries of Comparative Example 3 and Examples 1, 3 to 5 using lithium phosphate particles (LPO) having an average particle diameter D50 = 3.0 μm are examined. As can be understood from Table 2 and FIG. 10, in Comparative Example 3 corresponding to the holding period Tk = 0 minutes, the battery resistance ratio was 1.18, and the battery resistance was high. However, the batteries of Examples 3, 4, 1 (holding period Tk = 20, 40, 60 minutes) all had a lower battery resistance ratio (battery resistance) than Comparative Example 3. Moreover, when comparing the batteries of Examples 3, 4, and 1 (holding period Tk = 20, 40, 60 minutes), the battery resistance ratio (battery resistance) was lower as the battery had a longer holding period Tk. The reason is considered as follows. When the holding period Tk is short, the film 25 is generated, but its thickness is insufficient, and thereafter the film generation stops in a state where the nonaqueous electrolytic solution 40 (nonaqueous solvent) can be oxidatively decomposed. For this reason, after the second step, when the process proceeds to the third step and is switched to CC charging at a charge rate of 3.0 C, a large amount of oxidative decomposition of the nonaqueous solvent occurs at a time due to a large current. In addition to the film that has been generated, a thick film is formed. For this reason, resistance becomes high compared with the battery (battery of Example 1) which fully ensured holding | maintenance period Tk. However, oxidative decomposition of the non-aqueous solvent is suppressed during CC charging as compared with the battery of Comparative Example 3 that does not have the second step, as long as the coating film 25 is formed in the second step. For this reason, it is thought that battery resistance becomes small compared with a battery without the holding period Tk (Comparative Example 3) or a short battery (Example 3 with respect to Example 4). And in the battery of Example 1 (holding period Tk = 60 minutes) in which the holding period Tk was appropriately secured, the battery resistance ratio (battery resistance) could be lowered to the same level as in Comparative Example 6.

  On the other hand, when comparing the battery of Example 5 (holding period Tk = 90 minutes) and the battery of Example 1 (holding period Tk = 60 minutes), the battery resistance ratio changes even if the holding period Tk is increased. Do not (do not decline). When the coating film 25 is formed, it is difficult to cause oxidative decomposition of the non-aqueous solvent, and finally, no oxidative decomposition occurs. Then, since the coating film 25 is not generated, it is considered that the increase in the thickness of the coating film 25 is stopped and the increase in battery resistance is also eliminated. Therefore, it can be seen that even if the holding period Tk is unnecessarily increased, there is no effect of lowering the battery resistance, and it is preferable to end the second step in an appropriate holding period Tk and shift to the third step.

  Next, the batteries of Comparative Example 4 and Examples 6 to 9 using lithium phosphate particles (LPO) having an average particle diameter D50 = 1.5 μm are examined. These batteries can also be considered similarly to the batteries of Comparative Example 3 and Examples 1, 3 to 5 described above. That is, in Comparative Example 4 corresponding to the holding period Tk = 0 minutes, the battery resistance ratio = 1.12 and the battery resistance was high. However, the batteries of Examples 6 to 8 (holding period Tk = 10, 20, 30 minutes) all had a battery resistance ratio (battery resistance) lower than that of Comparative Example 4. Moreover, when the batteries of Examples 6 to 8 (holding period Tk = 10, 20, 30 minutes) are compared, the battery resistance ratio (battery resistance) decreases as the holding period Tk increases. In the battery of Example 8 (holding period Tk = 30 minutes), the battery resistance ratio (battery resistance) could be lowered to the same level as in Comparative Example 6. On the other hand, in the battery of Example 9 (holding period Tk = 60 minutes), compared with Example 8 (holding period Tk = 30 minutes), the battery resistance ratio does not change (does not decrease) even if the holding period Tk is increased. ).

  Furthermore, the batteries of Comparative Example 5 and Examples 10 to 13 using lithium phosphate particles (LPO) having an average particle diameter D50 = 0.8 μm are examined. These batteries can also be considered similarly to the batteries of Comparative Example 3, Examples 1, 3 to 5 and Comparative Examples 4 and 6 to 9 described above. That is, in Comparative Example 5 corresponding to the holding period Tk = 0 minutes, the battery resistance ratio = 1.09, and the battery resistance was high. However, the batteries of Examples 10 to 12 (holding period Tk = 10, 20, 30 minutes) all had a battery resistance ratio (battery resistance) lower than that of Comparative Example 5. Moreover, when the batteries of Examples 10 to 13 (holding period Tk = 10, 20, 30 minutes) are compared, the battery resistance ratio (battery resistance) decreases as the holding period Tk increases. And with the battery of Example 11 (holding period Tk = 20 minutes), the battery resistance ratio (battery resistance) could be lowered to the same level as in Comparative Example 6. Further, in the battery of Example 12 (holding period Tk = 30 minutes), the battery resistance ratio (battery resistance) was lower than that of Comparative Example 6 (battery resistance ratio = 0.99). However, in the battery of Example 13 (holding period Tk = 60 minutes), compared with Example 12 (holding period Tk = 30 minutes), the battery resistance ratio does not change (does not decrease) even if the holding period Tk is increased. ).

  From these results, it can be understood that the battery resistance can be lowered by providing the second step of holding the first voltage Vh for the holding period Tk compared to the case where the second step is not provided. In addition, the battery resistance can be lowered by increasing the holding period Tk. However, the holding period Tk has an appropriate length, and even if the holding period Tk is unnecessarily increased, the battery resistance does not decrease. Therefore, the second step is completed with the holding period Tk having an appropriate length, It can be seen that it is preferable to shift to three steps.

  From the above, as the holding period Tk, the second process is performed without extending the holding period Tk, and then the battery resistance Rn of the battery manufactured by performing the third process is replaced with the holding period Tk. When the first voltage Vh is held for the extended holding period extended by 1.5 times, and then compared with the battery resistance Re of the extended holding battery manufactured by performing the third step, Rn = 0.98Re˜1 It is recommended to select a period of .02Re. If the first voltage Vh is held for the holding period Tk selected in this way, the difference is no more than 2% compared to the battery resistance Re of the extended holding battery in which the holding period is extended by 1.5 times. The coating film 25 that becomes the battery resistance Rn can be formed. Therefore, if the first voltage Vh is held for such a holding period Tk, the formation of the coating film 25 is almost completed, and the process can proceed to the third step that immediately follows. That is, a thin coating 25 capable of preventing oxidative decomposition of the nonaqueous solvent is appropriately formed on the particle surface 24n of the positive electrode active material particles 24, and a battery with low battery resistance can be manufactured in a short time.

  Specifically, according to Table 2 and FIG. 10, the battery resistance in the case where the relationship between the holding period and the battery resistance is acquired in advance for the battery of the specific form to be manufactured and the holding period Tk is set to a certain value. Rn and the battery resistance Re when it is extended 1.5 times may be compared to find a range of the holding period Tk in which the battery resistance Rn is Rn = 0.98Re to 1.02Re. For example, the batteries of Comparative Example 3, Examples 1 and 3 to 5 (batteries indicated by ▪ in FIG. 10) using lithium phosphate particles (LPO) having an average particle diameter D50 = 3.0 μm are illustrated. The battery resistance Rn of the battery (Example 4) with the holding period Tk = 40 minutes is 1.02 in terms of the battery resistance ratio. On the other hand, the battery resistance Re of the battery (Example 1) having a holding period Tk = 60 minutes in which the holding period is extended by 1.5 times is 1.00 in terms of battery resistance. Therefore, the battery resistance Rn of the battery of Example 4 is included in Rn = 0.98Re to 1.02Re. From this, regarding the battery (Comparative Example 3, Examples 1, 3 to 5) using lithium phosphate particles (LPO) having an average particle diameter D50 = 3.0 μm, the retention period Tk = 40 minutes or more. I know what to do.

  Further, as the holding period Tk, the second process is performed without extending the holding period Tk, and then the battery resistance Rn of the battery manufactured by performing the third process and the holding period Tk are set to 1.5. When the first voltage Vh is held for the extended holding period extended twice, and then the battery resistance Re of the extended holding battery manufactured by performing the third step is compared, Rn = 0.99Re to 1.01Re More preferably, the period is selected. If the first voltage Vh is held for the holding period Tk selected in this way, the battery resistance Re of the extended holding battery whose holding period is extended by 1.5 times differs by no more than 1%. The coating film 25 that becomes the battery resistance Rn can be formed. Therefore, if the first voltage Vh is held for such a holding period Tk, the formation of the film 25 is almost completed and the process can proceed to the subsequent third step. That is, a thin coating 25 that can prevent oxidative decomposition of the nonaqueous solvent is further appropriately formed on the particle surface 24n of the positive electrode active material particles 24, and a battery with low battery resistance can be manufactured in a short time.

  In this case, for example, regarding batteries (Comparative Example 3, Examples 1, 3 to 5) using lithium phosphate particles (LPO) having an average particle diameter D50 = 3.0 μm, the retention period Tk = 50 minutes. It can be seen that the above is sufficient.

  Next, the relationship between the average particle diameter D50 of the lithium phosphate particles 28 added to the positive electrode active material layer 23 and the retention period Tk is examined (see Table 2 and FIG. 11). First, among batteries (Comparative Example 3, Examples 1 and 3 to 5) using lithium phosphate particles 28 having an average particle diameter D50 = 3.0 μm, the battery resistance ratio was 1.00 (battery of Comparative Example 6). The battery holding period Tk of the battery resistance equivalent to that of Example 1 is the holding period Tk = 60 minutes in the first embodiment. Further, in the battery using the particles 28 having the average particle diameter D50 = 1.5 μm (Comparative Example 4, Examples 6 to 9), the retention period Tk of Example 8 is 30 minutes. Further, in the battery using the particles 28 having the average particle diameter D50 = 0.8 μm (Comparative Example 5 and Examples 10 to 13), the holding period Tk of Example 11 is 20 minutes. FIG. 11 shows a graph of the relationship between the average particle diameter D50 of the particles 28 and the retention period Tk when the battery resistance ratio is 1.00 in these batteries. That is, the size of the holding period Tk required to achieve the same battery resistance (battery resistance ratio 1.00) as that of the battery of Comparative Example 6 in which the initial charge was CC charged at a small charge rate of 0.33 C is It has a high correlation with the average particle diameter D50 of the lithium acid particles 28, specifically, a linear relationship. The smaller the average particle diameter D50 of the particles 28, the shorter the retention period Tk. In particular, when the average particle diameter D50 of the lithium phosphate particles 28 is set to D50 = 1.5 μm or less, the holding period Tk can be set to Tk = 30 minutes or less. Can contribute. If the addition amount is the same, the smaller the average particle diameter D50, the more the number and surface area of the lithium phosphate particles 28 increase. Therefore, the reaction with the generated hydrogen fluoride (HF) is likely to occur. This is probably because the coating film 25 was formed in a short time.

As described above, in the method for manufacturing the battery 1, in the initial charging step, after the first step, the terminal voltage Vt is changed to the first in the lower decomposition area Ad over the predetermined holding period Tk once in the second step. The voltage Vh is maintained, and then charging is performed until the second voltage Ve is reached in the third step.
For this reason, in the second step, oxidative decomposition of the nonaqueous electrolytic solution 40 occurs while the inter-terminal voltage Vt is maintained at the first voltage Vh (Vt = Vh). However, the first voltage Vh is set to a voltage within a low voltage range called the lower decomposition region Ad even in a range where the nonaqueous electrolytic solution 40 undergoes oxidative decomposition. For this reason, the oxidative decomposition of the nonaqueous electrolytic solution 40 occurs only gradually, and the coating film 25 containing fluorine and phosphorus can be formed thinly on the particle surface 24n of the positive electrode active material particles 24, and the battery resistance can be kept low.

(Embodiment 2)
Next, a second embodiment of the present invention will be described. In the first embodiment described above, in the first charging step of the battery 1, CC charging at the charging rate of 3.0 C is performed in the first step S1 until the terminal voltage Vt reaches the first voltage Vh (Vt = Vh = 4.1V). )went. Thereafter, in the second step S2, CV charging is performed over a predetermined holding period Tk, and then CC charging at a charging rate of 3.0C is performed in the third step S3 until the inter-terminal voltage Vt reaches the second voltage Ve. (Vt = Ve = 4.9V).

On the other hand, in the second embodiment (the same applies to the modified embodiment), in the first step SA1 of the initial charging step of the battery 1, as in the first embodiment, the inter-terminal voltage Vt reaches the first voltage Vh as in the first embodiment. (Vt = Vh = 4.1V) (see FIG. 12). However, the difference is that the charge rate is 5.0 C (charge current Ib = 5.0 C). Also, in the third step SA3, as in the first embodiment, CC charging was performed until the inter-terminal voltage Vt reached the second voltage Ve (Vt = Ve = 4.9V). However, the difference is that the charge rate is 5.0 C (charge current Ib = 5.0 C). Further, the second voltage Ve = 4.75V, that is, the CC charging is performed until the inter-terminal voltage Vt reaches Ve = 4.75V.
Further, in the second step SA2, the holding period Tk is not determined, but the CV charging for maintaining the inter-terminal voltage Vt at the first voltage Vh (= 4.1V) is determined in advance by the charging current Ib flowing through the battery 1. The difference is that the process is performed until the cut-off current value Ibc is reached. In the second embodiment, the cutoff current value Ibc = 0.05C.
Therefore, the following description will focus on the parts different from the first embodiment, and the same parts will be omitted or simplified.

  In the second embodiment, the same battery 1 as in the first embodiment was used. Further, the manufacturing illegality of the battery 1 is the same as that of the first embodiment except for the initial charging step described below. However, as the lithium phosphate particles 28 added to the positive electrode active material layer 23, particles having an average particle diameter D50 of D50 = 3.0 μm (or 1.5 μm or 0.8 μm) were used in the first embodiment. Form 2 is different in that particles having an average particle diameter D50 of D50 = 1.0 μm are used.

  Next, in the manufacturing method of the battery 1 according to the second embodiment, the initial charging step will be described with reference to FIGS. In the initial charging step, first, the battery 1 is connected to a CC-CV charging / discharging device (not shown). As shown in FIGS. 12 and 14, after the start time t0, the battery 1 is charged at a charging rate of 5.0 C (charging current Ib1 CC charge with a current of = 5.0 C), and the inter-terminal voltage Vt is increased to Vt = 4.1 V (first voltage Vh) (first step SA1). The charging current Ib1 (= 5.0 C) at the end of the first step (1-2 switching time t12, which is the timing when the inter-terminal voltage Vt = Vh (= 4.1 V)) is set as the final current value. However, since the first step in the second embodiment is constant current charging, as described above, the final current value is equal to the charging current Ib1 in the first step. As shown in FIG. 14, in the second embodiment, the period of the first step SA1 (t0 to t12) is about 1 minute long, and immediately after the CC charging start time t0 in the first step SA1. The inter-terminal voltage Vt rises to about 3V, and then the inter-terminal voltage Vt reaches the first voltage Vh (= 4.1V) in about 1 minute.

  Next, in the second step (SA2), CV charging is performed while the inter-terminal voltage Vt = Vh (= 4.1V) is maintained. Specifically, as shown in FIG. 13, after 1-2 switching time t12, the charging current Ib2 is detected in step SA21, and in step SA22, the charging current Ib2 becomes the cut-off current value Ibc = 0.05C or less. It is determined whether or not. If No, that is, if the charging current Ib2 is larger than 0.05C (Ib2> 0.05C), the process returns to step SA21. On the other hand, if Yes in step SA22, that is, if the charging current Ib2 becomes 0.05C or less (Ib2 ≦ 0.05C) (this timing is set to 2-3 switching time t23), the third step of FIG. Proceed to SA3.

In the second embodiment, the period (t12 to t23) of the second step SA2 is about 21 minutes long. Immediately after the start of CV charging in the second step SA2, the charging current Ib2 rapidly decreases from the final current value Ib1, then gradually decreases, and then gradually approaches Ib2 = 0. The shape of the curve, as shown in FIG. 14, a shape similar to the graph of y = 1-e ^x. This is considered due to the following reasons. At the beginning of the second step SA2, the voltage Vt between the terminals is set to the first voltage Vh (= 4.1 V) in the lower decomposition zone Ad (4.0 to 4.4 V), so that the oxidation of the nonaqueous electrolytic solution 40 is performed. Decomposition occurs one after another, and a large current flows as the decomposition current. However, with the passage of time, the lithium phosphate particles 28 contained in the positive electrode active material layer 23 are consumed, the coating 25 is formed, and the oxidative decomposition of the nonaqueous electrolytic solution 40 is suppressed, so that the charging current Ib2 is reduced. It is thought that it will gradually decrease.

  In the subsequent third step SA3, as shown in FIG. 12, the battery 1 is CC-charged at a charge rate of 5.0C (charge current Ib3 = 5.0C), and the inter-terminal voltage Vt becomes Vt = 4.75V. When the voltage rises to (second voltage Ve), the initial charging process ends (this timing is set as end time t3e). In the second embodiment, the period (t23 to t3e) of the third step SA3 is about 10 minutes long. Therefore, the initial charging step in the second embodiment can be completed in a total of about 32 minutes (= 1 + 21 + 10) from the first step to the third step.

In the second step SA2 of the second embodiment, the inter-terminal voltage Vt of the battery 1 is held at the first voltage Vh (= 4.1 V). However, variations in the individual batteries 1 cause differences in the rate of film formation formed on the surface 24n of the positive electrode active material particles 24 (see FIG. 4). Therefore, when the period (t12 to t23) for holding the first voltage Vh is the same, that is, when the holding period Tk is set as in the first embodiment, the surface 24n of the positive electrode active material particles 24 is formed. A state such as a thickness of the formed film 25 is different, and a difference such as a variation in battery resistance occurs. For this reason, in order to obtain a coating film 25 having an appropriate thickness in any of the batteries 1, it is necessary to set a longer holding period Tk in accordance with a battery having a slow coating generation rate. For example, the holding period Tk may be too long.
On the other hand, in the manufacturing method of the second embodiment, in the second step SA2, the first voltage Vh is held until the charging current Ib2 becomes equal to or lower than the cutoff current value Ibc (Ib2 ≦ Ibc). Even in the case where each of the batteries 1 is present, the coating 25 having the same thickness can be formed on the surface 24n of the positive electrode active material particle 24 in the shortest time. For this reason, when the initial charging process is performed in order for a large number of batteries 1, it is possible to suppress variations in the thickness of the coating film 25 in each battery, and consequently variations in battery resistance, while reducing the overall process.

  Next, for each battery 1, the cut-off current value Ibc in the initial charging process is changed to five levels of Ibc = 2.0C, 0.5C, 0.1C, 0.05C, 0.02C, and the initial charging process. The battery resistance (IV resistance) was measured in the same manner as in the first embodiment for the battery 1 that finished the process. Furthermore, the battery resistance of each battery is determined with reference to the battery resistance of the battery when the second step SA2 is not provided (in short, when the battery 1 is charged by CC charging at a charge rate of 5.0 C). A ratio Rr was obtained (see FIG. 15).

  According to the graph of FIG. 15, the second step is provided as compared with the reference battery (Rr = 1.00) in which the initial charging step is performed by CC charging at the charging rate of 5.0 C, and the cutoff current value Ibc is set. It can be seen that the battery resistance ratio Rr decreases with decreasing. For example, when Ibc = 2.0C, the battery resistance ratio Rr = 93%, and it can be seen that the battery resistance can be lowered by about 7%. Further, when Ibc = 1.0 C, the battery resistance ratio Rr = 90%, and it can be seen that the battery resistance can be lowered by about 10%. Further, when Ibc = 0.5C, the battery resistance ratio Rr = 88%, and it can be seen that the battery resistance can be reduced by about 12%. Furthermore, when Ibc = 0.05C, the battery resistance ratio Rr = 84.5%, which indicates that the battery resistance can be reduced by about 15%.

  However, even if the cut-off current value Ibc is smaller than 0.05C (for example, Ibc = 0.02C), the battery resistance ratio Rr does not decrease as compared with the case where Ibc = 0.05C. That is, it can be seen that even if the cut-off current value Ibc is smaller than 0.05C, the battery resistance cannot be further reduced. This is because the formation of the coating 25 is because almost all of the lithium phosphate particles 28 contained in the positive electrode active material layer 23 are consumed when the charging current Ib reaches Ib = 0.05C. Conceivable.

  Thus, in the manufacturing method of the second embodiment, the final current value Ib1 in the first step SA1 is Ib1 = 5.0C which is 1C or more, whereas the cut-off current value Ibc is equal to the final current value Ib1. The second step SA2 is performed until Ibc = 0.05C, which is sufficiently small (1/100). If the second step is performed until the cut-off current value becomes 0.05 C, the battery resistance is almost the same as when the cut-off current value is made smaller than this (for example, when the cut-off current value is 0.02 C). It becomes. Therefore, when the cut-off current value Ibc is Ibc = 0.05C, almost the entire amount of the coating film 25 containing fluorine and phosphorus formed on the particle surface 24n of the positive electrode active material particles 24 in the shortest time is obtained in the second step SA2. Can be formed. In addition, a good coating film 25 can be formed, and the battery resistance can be lowered (specifically, about 15% smaller) than when the second step SA2 is not provided.

(Deformation)
In the second embodiment, the cutoff current value Ibc in the second process SA2 (step SA22) is set to Ibc = 0.05C. However, although the effect of lowering the battery resistance is suppressed, the cut-off current value Ibc can be made larger than 0.05C in order to shorten the period of the second step SA2. That is, the present modification is different from the second embodiment only in that the cut-off current value Ibc is set to Ibc = 2.0C, and the others are the same.

  Of the method for manufacturing the battery 1 according to this modification, the initial charging step will be described with reference to FIGS. 12 and 14 to 16. First, in the first step (SA1), in the same manner as in the first embodiment, from the start time t0 to the 1-2 switching time t12, the battery 1 is charged with a current having a charge rate of 5.0C (charge current Ib1 = 5.0C). Charging is performed to increase the inter-terminal voltage Vt to Vt = 4.1 V (first voltage Vh). The final current value is equal to the charging current Ib1 = 5.0C.

In the subsequent second step (SA2a), as in the first and second embodiments, CV charging is performed while maintaining the inter-terminal voltage Vt = Vh (= 4.1 V). Specifically, as shown in FIG. 16, the charging current Ib2 is detected in step SA21 after the 1-2 switching time t12. Next, in step SA22a, a cutoff current value Ibc = 2.0C larger than that of the second embodiment (Ibc = 0.05C) is used, and it is determined whether or not the charging current Ib2 is equal to or less than Ibc = 2.0C. If No, that is, if the charging current Ib2 is larger than 2.0C (Ib2> 2.0C), the process returns to step SA21. On the other hand, if Yes in step SA22a, that is, if the charging current Ib2 becomes 2.0C or less (Ib2 ≦ 2.0C) (this timing is defined as 2-3 switching time t23a, see FIG. 14), FIG. Proceed to the third step SA3.
The period (t12 to t23a) of the second step SA2a in this variation is about 1 minute long. In the case of this modification, the period from t23a to t23 does not exist in FIG.

In the subsequent third step SA3, as in the second embodiment, as shown in FIG. 12, the battery 1 is CC-charged with a current having a charge rate of 5.0 C, and the inter-terminal voltage Vt is Vt = Ve = 4.75V. When it rises to the end, the initial charging process is terminated. In the second embodiment, the period of the third step SA3 (t23a to t3e) is also about 10 minutes long.

  Eventually, the initial charging process of this modification can be completed in about 12 minutes (= 1 + 1 + 10) in total from the first process to the third process, which is significantly shorter than that of the second embodiment. Further, as shown in FIG. 15, in the battery 1 of the present modification in which the cut-off current value Ibc = 2.0C, the battery resistance is lowered by about 7% compared to the case where the second step is not provided. be able to.

  In this modification, Ibc = 2.0C is used as the cut-off current value in step SA22a. However, for example, the second step can be performed using Ibc = 1.0C as the cut-off current value. . In this case, the period of the second step (t12 to t23b) is about 2 minutes. The 2-3 switching time t23b is a timing when the charging current Ib2 becomes 1.0C or less (Ib2 ≦ 1.0C) (see FIG. 14). Also in this case, the period from t23b to t23 in FIG. 14 does not exist. Thus, when the cut-off current value Ibc = 1.0 C, the initial charging step (first step to third step) can be completed in about 13 minutes (= 1 + 2 + 10). On the other hand, by setting the cut-off current value Ibc = 1.0 C, the battery resistance can be reduced by about 10% compared to the case where the second step is not provided.

  Also in the manufacturing method of the battery 1 described in the above-described Embodiment 2 and the modification, the lithium phosphate particles 28 included in the positive electrode active material layer 23 are particles having an average particle diameter D50 of D50 ≦ 1.5 μm. For this reason, if the addition amount is the same, the total number of particles and the surface area increases, so that the reaction with the generated hydrofluoric acid is likely to occur, the coating 25 can be formed in a short time, and the time required for the second step SA2, SA2a, As a result, the time required for the initial charging process can be shortened.

  The batteries 1 according to the manufacturing methods described in the first and second embodiments and the modifications are all at least partly within the range of SOC = 0 to 100%, and the positive electrode potential Ep is 4.5 V (vs. Li / Li +) or higher potential. For this reason, when the SOC of the battery 1 is high, the nonaqueous electrolytic solution 40 is oxidatively decomposed on the particle surface 24n of the positive electrode active material particles 24 and easily generates hydrogen ions. Moreover, since the nonaqueous electrolytic solution 40 includes the compound 41 containing fluorine, hydrofluoric acid is easily generated from hydrogen ions and fluorine. However, in the manufacturing method of the battery 1 according to the first and second embodiments and the modified embodiment, the coating containing fluorine and phosphorus on the particle surface 24n of the positive electrode active material particles 24 in the second step S2, SA2, SA2a in the initial charging step. 25, the non-aqueous electrolyte 40 can be appropriately prevented from being oxidatively decomposed after the initial charging step.

Furthermore, in the manufacturing method according to the first and second embodiments and the modified embodiment, the first process S1, SA1 and the third process S3, SA3 are CC charged with the current values Ib1, Ib3 of 3.0C or 5.0C. Thereby, the time concerning a 1st process and a 3rd process can also be shortened, and an initial charge process can be further shortened.
In any of the first and second embodiments and the modified embodiments, the charging current Ib1 in the first step and the charging current Ib3 in the third step are the same. However, the charging currents Ib1 and Ib3 may have different sizes, for example, Ib1 = 3.0C and Ib3 = 5.0C. In particular, Ib1 ≦ Ib3 is preferable. As can be easily understood with reference to FIG. 14, the contribution to shortening the charging time t by increasing the charging current is greater for Ib3 than for Ib1. Further, when Ib1 is reduced, the time of the first step increases, but there is also an advantage that the amount of the coating film 25 formed in the first step can be suppressed before shifting to the second step.

In the above, the present invention has been described according to the first and second embodiments and the modified embodiments. However, the present invention is not limited to the above-described embodiments and the like, and may be appropriately changed without departing from the gist thereof. Needless to say, it can be applied.
For example, in the battery 1 and the like described above, the lithium phosphate particles 28 are used as the metal phosphate particles, but the present invention is not limited to this. For example, other metal phosphate particles such as sodium phosphate, potassium phosphate, magnesium phosphate, and calcium phosphate may be added to the positive electrode active material layer. Further, metal pyrophosphates such as lithium pyrophosphate particles, sodium pyrophosphate particles, magnesium pyrophosphate particles, calcium pyrophosphate particles instead of or together with metal phosphate particles such as lithium phosphate particles 28 You may add particle | grains to a positive electrode active material layer.

1 Lithium ion secondary battery (battery)
20 Electrode body 21 Positive electrode plate (positive electrode)
22 Positive electrode current collector foil 23 Positive electrode active material layer 24 Positive electrode active material particle 24n Particle surface 25 Coating 28 Lithium phosphate particle (metal phosphate particle)
31 Negative electrode plate (negative electrode)
40 Non-aqueous electrolyte 41 Compound containing fluorine 42 Metal phosphate particle Ep Positive electrode potential En Negative electrode potential Tk Holding period Vt Terminal voltage Vh First voltage Ad Lower decomposition zone Epd Decomposition lower limit potential Vtd Decomposition lower limit voltage Ve Second voltage S1 , SA1 First step S2, SA2 Second step S3, SA3 Third step Ib, Ib1, Ib2, Ib3 Charging current Ib1 (first step) final current value Ibc Cut-off current value

Claims (9)

A positive electrode having a positive electrode active material layer containing positive electrode active material particles;
A negative electrode,
A non-aqueous electrolyte solution containing a fluorine-containing compound,
The positive electrode active material particle is a method for producing a lithium ion secondary battery having a film containing fluorine and phosphorus on the particle surface,
The positive electrode active material layer includes particles of at least one of metal phosphate and metal pyrophosphate,
The process of charging the lithium ion secondary battery for the first time is as follows:
A first step of charging the lithium ion secondary battery to raise the voltage of the lithium ion secondary battery to a first voltage in a lower decomposition region;
A second step of maintaining the voltage of the lithium ion secondary battery at the first voltage;
After the second step a, a third step of charging with a charging current larger than 1C up to a second voltage higher than the first voltage, a method for manufacturing a lithium ion secondary battery. It is a manufacturing method of the lithium ion secondary battery according to claim 1,
The second step includes
A method for manufacturing a lithium ion secondary battery, wherein the voltage of the lithium ion secondary battery is maintained at the first voltage over a predetermined holding period. It is a manufacturing method of the lithium ion secondary battery according to claim 2,
The retention period is
The battery resistance Rn of the battery manufactured by performing the second step without extending the holding period and then performing the third step;
Instead of the holding period, the first voltage is held for an extended holding period obtained by extending the holding period by a factor of 1.5, and then the battery resistance Re of the extended holding battery manufactured by performing the third step is as follows. When comparing
A method for manufacturing a lithium ion secondary battery in a period in which Rn = 0.98 Re to 1.02 Re. It is a manufacturing method of the lithium ion secondary battery according to claim 2,
The retention period is
The battery resistance Rn of the battery manufactured by performing the second step without extending the holding period and then performing the third step;
Instead of the holding period, the first voltage is held for an extended holding period obtained by extending the holding period by a factor of 1.5, and then the battery resistance Re of the extended holding battery manufactured by performing the third step is as follows. When comparing
A method for manufacturing a lithium ion secondary battery in a period in which Rn = 0.99Re to 1.01Re. It is a manufacturing method of the lithium ion secondary battery according to claim 1,
The second step includes
The manufacturing method of the lithium ion secondary battery hold | maintained at a said 1st voltage until the magnitude | size of the charging current of the said lithium ion secondary battery becomes below a predetermined cut-off current value. It is a manufacturing method of the lithium ion secondary battery according to claim 5,
The method of manufacturing a lithium ion secondary battery, wherein the cutoff current value is 2/5 of the final current value at the end of the first step. It is a manufacturing method of the lithium ion secondary battery according to claim 5,
The final current value at the end of the first step is 1C or more,
The method of manufacturing a lithium ion secondary battery, wherein the cutoff current value is 0.05C. It is a manufacturing method of the lithium ion secondary battery of any one of Claims 1-7,
At least one of the metal phosphate and metal pyrophosphate contained in the positive electrode active material layer,
A method for producing a lithium ion secondary battery having an average particle diameter of 1.5 μm or less. It is a manufacturing method of the lithium ion secondary battery according to any one of claims 1 to 8,
The first step and the third step are:
A method of manufacturing a lithium ion secondary battery that is charged with a constant current at a predetermined current value of 3C or more.

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