JP2012212513A - State detection method of lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a state detection method capable of simply and highly accurately detecting a state of a lithium secondary battery.SOLUTION: A state detection method for detecting a state of a lithium secondary battery includes: a discharge step of discharging the battery to SOC10% or lower; a measurement step of measuring an impedance of the battery discharged by the discharge step; and a state detection step of detecting a battery state based on measured values of the impedance obtained by the measurement step.

Description

  The present invention relates to a state detection method for detecting a state of a battery. In particular, the present invention relates to a state detection method suitable for a lithium secondary battery.

  In recent years, lithium secondary batteries, nickel hydride batteries, and other secondary batteries have become increasingly important as power sources for mounting on vehicles or as power sources for personal computers and portable terminals (for example, Patent Document 1). In particular, a lithium secondary battery (typically a lithium ion secondary battery) that is lightweight and obtains a high energy density is expected to be preferably used as a large-sized power source for mounting on a vehicle. By the way, as a method for detecting the state of this type of lithium secondary battery, for example, Japanese Unexamined Patent Application Publication No. 2009-244088 (Patent Document 1) is disclosed. This publication describes that an alternating voltage or alternating current of a specific frequency is input to a lithium ion secondary battery, and the state of the lithium ion secondary battery is detected based on the phase difference of the output with respect to the input. Japanese Patent Laid-Open No. 2000-299137 (Patent Document 2) is cited as another technique that utilizes the measurement of the impedance of a battery.

JP 2009-244088 A JP 2000-299137 A

  In the technique disclosed in Patent Document 1 described above, an AC voltage or an AC current having a specific frequency is input to the lithium ion secondary battery, and the state of the lithium ion secondary battery (in the negative electrode) is based on the phase difference of the output with respect to the input. (Deposition state of metallic lithium). According to the publication, a higher battery voltage is preferable because a change in phase difference becomes larger. However, with this technique, there is a limit to the detection accuracy of lithium deposition, and there are cases where accurate inspection cannot be performed. The present invention proposes a novel method for detecting the state of the lithium secondary battery (lithium deposition state at the negative electrode).

  The method according to the present invention is a state detection method for detecting the state of a lithium secondary battery. The state detection method includes a discharging step of discharging the battery to 10% or less of SOC (State Of Charge), a measuring step of measuring the impedance of the battery discharged by the discharging step, and the impedance obtained in the measuring step. And a state detection step of detecting the state of the battery based on the measured value.

  According to the lithium secondary battery state detection method of the present invention, when the SOC is a predetermined value of 10% or less (for example, 5%, preferably 0%) in the discharging step, the impedance of the battery 100 is measured. The state can be detected easily and with high accuracy. That is, in the low charge state where the SOC is 10% or less, the measured impedance value greatly changes depending on the presence or absence of lithium deposition on the negative electrode. For this reason, it is possible to appropriately detect the lithium deposition state (battery deterioration state) at the negative electrode by using the measured impedance value obtained in the low charge state.

  In a preferred aspect of the lithium secondary battery state detection method disclosed herein, the impedance is measured at a plurality of frequencies in the measurement step. In this case, it is preferable to plot the real part and the imaginary part of the impedance measured at the plurality of frequencies on a plane coordinate and calculate the reaction resistance value from the obtained impedance circle. By utilizing such reaction resistance value, it is possible to detect the lithium deposition state (battery deterioration state) at the negative electrode simply and with high accuracy.

  In a preferable aspect of the lithium secondary battery state detection method disclosed herein, in the state detection step, the reaction resistance value calculated in the measurement step is compared with a predetermined threshold (determination criterion for deterioration). By doing so, it is determined whether or not the battery has deteriorated. At this time, by preparing a plurality of determination criteria (threshold values) for the deterioration, the degree of deterioration of the battery can be determined stepwise. In a preferred embodiment, the threshold value is 2 more than the initial reaction resistance value of the battery (that is, the reaction resistance value in an initial state where the battery has not deteriorated (typically before use in a normal use mode)). It is set to a value that is at least twice as large. By setting a value that is twice or more larger than the initial reaction resistance value as the threshold value, it is possible to prevent erroneous detection and improve the detection accuracy of battery deterioration.

It is a figure which shows typically the lithium secondary battery which concerns on one Embodiment of this invention. It is a figure which shows typically the wound electrode body which concerns on one Embodiment of this invention. It is a figure which shows the Cole-Cole plot which concerns on one Embodiment of this invention. It is a figure for demonstrating a Cole-Cole plot and an equivalent circuit. It is a block diagram which shows the lithium secondary battery system which concerns on one Embodiment of this invention. It is a side view of the vehicle carrying the lithium secondary battery system which concerns on one Embodiment of this invention. It is a figure which shows typically the lithium secondary battery which concerns on one test example of this invention. It is a figure which shows the Cole-Cole plot obtained by SOC0%. It is a figure which shows the Cole-Cole plot obtained by SOC20%. It is a figure which shows the Cole-Cole plot obtained by SOC40%. It is a figure which shows the Cole-Cole plot obtained by SOC60%. It is a figure which shows the Cole-Cole plot obtained by SOC80%. It is a graph which shows the relationship between reaction resistance value and SOC.

  Embodiments according to the present invention will be described below with reference to the drawings. In the following drawings, members / parts having the same action are described with the same reference numerals. Note that the dimensional relationship (length, width, thickness, etc.) in each drawing does not reflect the actual dimensional relationship. Further, matters other than the matters specifically mentioned in the present specification and matters necessary for the implementation of the present invention (for example, the configuration and manufacturing method of an electrode body including a positive electrode and a negative electrode, the configuration and manufacturing method of a separator and an electrolyte, General techniques relating to the construction of lithium secondary batteries and other batteries, etc.) can be understood as design matters for those skilled in the art based on the prior art in this field.

  Hereinafter, a method for detecting a state of a lithium secondary battery according to an embodiment of the present invention will be described.

  This state detection method is performed on the lithium secondary battery 100. A lithium secondary battery 100 (hereinafter referred to as “battery” where appropriate) includes, for example, a long positive electrode sheet 10 and a long negative electrode sheet 20 with a long separator 40 as shown in FIG. A flatly wound electrode body (winding electrode body) 80 is accommodated in a case (flat box shape) case 50 that can accommodate the wound electrode body 80 together with a non-aqueous electrolyte (not shown). It has the structure made.

  The case 50 includes a flat rectangular parallelepiped case main body 52 having an open upper end, and a lid 54 that closes the opening. As a material constituting the case 50, a metal material such as aluminum or steel is preferably used (in this embodiment, aluminum). Or the case 50 formed by shape | molding resin materials, such as a polyphenylene sulfide (PPS) and a polyimide resin, may be sufficient. On the upper surface of the case 50 (that is, the lid body 54), a positive electrode terminal 70 that is electrically connected to the positive electrode of the wound electrode body 80 and a negative electrode terminal 72 that is electrically connected to the negative electrode 20 of the electrode body 80 are provided. Yes. In the case 50, a flat wound electrode body 80 is accommodated together with a non-aqueous electrolyte (not shown).

  The wound electrode body 80 according to the present embodiment is a wound electrode body of a normal lithium secondary battery except for the configuration of a layer (positive electrode active material layer) containing an active material provided in the positive electrode sheet 10 described later. Similarly, as shown in FIG. 2, a long (strip-shaped) sheet structure is provided in a stage before assembling the wound electrode body 80.

  The positive electrode sheet 10 has a positive electrode active material layer 14 containing a positive electrode active material on both surfaces of a long sheet-like foil-shaped positive electrode current collector (hereinafter referred to as “positive electrode current collector foil”) 12. Have a structure. However, the positive electrode active material layer 14 is not attached to one side edge (the left side edge portion in the drawing) along the widthwise end of the positive electrode sheet 10, and the positive electrode current collector 12 is exposed with a certain width. A positive electrode active material layer non-formed part is formed.

  Similarly to the positive electrode sheet 10, the negative electrode sheet 20 holds a negative electrode active material layer 24 containing a negative electrode active material on both sides of a long sheet-like foil-shaped negative electrode current collector (hereinafter referred to as “negative electrode current collector foil”) 22. Has a structured. However, the negative electrode active material layer 24 is not attached to one side edge (the right side edge portion in the drawing) along the widthwise edge of the negative electrode sheet 20, and the negative electrode current collector 22 is exposed with a certain width. A negative electrode active material layer non-formed portion is formed.

  In producing the wound electrode body 80, the positive electrode sheet 10 and the negative electrode sheet 20 are laminated via the separator sheet 40. At this time, the positive electrode sheet 10 and the negative electrode sheet 20 are formed such that the positive electrode active material layer non-formed portion of the positive electrode sheet 10 and the negative electrode active material layer non-formed portion of the negative electrode sheet 20 protrude from both sides in the width direction of the separator sheet 40. Are overlapped slightly in the width direction. The laminated body thus stacked is wound, and then the obtained wound body is crushed from the side surface direction and ablated, whereby a flat wound electrode body 80 can be produced.

  A wound core portion 82 (that is, the positive electrode active material layer 14 of the positive electrode sheet 10, the negative electrode active material layer 24 of the negative electrode sheet 20, and the separator sheet 40) is densely arranged in the central portion of the wound electrode body 80 in the winding axis direction. Laminated portions) are formed. Moreover, the electrode active material layer non-formation part of the positive electrode sheet 10 and the negative electrode sheet 20 protrudes outward from the winding core part 82 at both ends in the winding axis direction of the wound electrode body 80, respectively. A positive electrode lead terminal 74 and a negative electrode lead terminal 76 are provided on the positive electrode side protruding portion (that is, the non-formed portion of the positive electrode active material layer 14) 84 and the negative electrode side protruding portion (that is, the non-formed portion of the negative electrode active material layer 24) 86, respectively. Attached and electrically connected to the positive terminal 70 and the negative terminal 72 described above.

  The constituent elements of the wound electrode body 80 may be the same as those of the wound electrode body of the conventional lithium secondary battery except for the positive electrode sheet 10, and are not particularly limited. For example, the negative electrode sheet 20 can be formed by applying a negative electrode active material layer 24 mainly composed of a negative electrode active material for a lithium secondary battery on a long negative electrode current collector 22. For the negative electrode current collector 22, a copper foil or other metal foil suitable for the negative electrode is preferably used. As the negative electrode active material, one or more of materials conventionally used in lithium secondary batteries can be used without any particular limitation. Preferable examples include carbon-based materials such as graphite carbon and amorphous carbon, lithium-containing transition metal oxides and transition metal nitrides, Si-based materials, and Sn-based materials.

  The positive electrode sheet 10 can be formed by applying a positive electrode active material layer 14 mainly composed of a positive electrode active material for a lithium secondary battery on a long positive electrode current collector 12. For the positive electrode current collector 12, an aluminum foil or other metal foil suitable for the positive electrode is preferably used.

As the positive electrode active material, one type or two or more types of materials conventionally used in lithium secondary batteries can be used without any particular limitation. As a preferable application object of the technology disclosed herein, lithium and a transition metal element such as lithium nickel oxide (LiNiO ₂ ), lithium cobalt oxide (LiCoO ₂ ), and lithium manganese oxide (LiMn ₂ O ₄ ) are used. A positive electrode active material mainly containing an oxide containing a constituent metal element (lithium transition metal oxide) can be given. Among them, a positive electrode active material (typically, substantially a lithium nickel cobalt manganese composite oxide substantially composed of lithium nickel cobalt manganese composite oxide (for example, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ). Application to a positive electrode active material comprising:

  Here, the lithium nickel cobalt manganese composite oxide is an oxide having Li, Ni, Co, and Mn as constituent metal elements, and at least one other metal element in addition to Li, Ni, Co, and Mn (that is, It also includes oxides containing transition metal elements and / or typical metal elements other than Li, Ni, Co, and Mn. The metal element is, for example, one or two selected from the group consisting of Al, Cr, Fe, V, Mg, Ti, Zr, Nb, Mo, W, Cu, Zn, Ga, In, Sn, La, and Ce. It can be more than a seed element. The same applies to lithium nickel oxide, lithium cobalt oxide, and lithium manganese oxide.

  As such a lithium transition metal oxide (typically in particulate form), for example, a lithium transition metal oxide powder prepared by a conventionally known method can be used as it is. For example, lithium transition metal oxide powder substantially composed of secondary particles having an average particle diameter in the range of about 1 μm to 25 μm can be preferably used as the positive electrode active material.

  The positive electrode active material layer 14 can contain one kind or two or more kinds of materials that can be used as components of the positive electrode active material layer in a general lithium secondary battery, if necessary. An example of such a material is a conductive material. As the conductive material, a carbon material such as carbon powder or carbon fiber is preferably used. Alternatively, conductive metal powder such as nickel powder may be used. In addition, as a material that can be used as a component of the positive electrode active material layer, various polymer materials that can function as a binder (binder) of the above-described constituent materials can be given.

  Although not particularly limited, the ratio of the positive electrode active material to the entire positive electrode active material layer is preferably about 50% by mass or more (typically 50 to 95% by mass), preferably about 75 to 90% by mass. Preferably there is. In the positive electrode active material layer having a composition containing a conductive material, the proportion of the conductive material in the positive electrode active material layer can be, for example, 3 to 25% by mass, and preferably about 3 to 15% by mass. Moreover, when it contains positive electrode active material layer formation components (for example, polymer material) other than a positive electrode active material and a electrically conductive material, it is preferable that the total content rate of these arbitrary components shall be about 7 mass% or less, and about 5 mass%. The following (for example, about 1 to 5% by mass) is preferable.

  As the method for forming the positive electrode active material layer 14, a positive electrode active material layer forming paste in which a positive electrode active material (typically granular) and other positive electrode active material layer forming components are dispersed in an appropriate solvent (preferably an aqueous solvent). Preferably, a method of coating the electrode collector on one side or both sides (here, both sides) of the positive electrode current collector 12 and drying it can be preferably employed. After drying the positive electrode active material layer forming paste, an appropriate press treatment (for example, various conventionally known press methods such as a roll press method, a flat plate press method, etc. can be employed) is carried out, whereby the positive electrode active material layer The thickness and density of 14 can be adjusted.

  Suitable separator sheets 40 used between the positive and negative electrode sheets 10 and 20 include those made of a porous polyolefin resin. For example, a porous separator sheet made of synthetic resin (for example, made of polyolefin such as polyethylene) can be suitably used. When a solid electrolyte or a gel electrolyte is used as the electrolyte, there may be a case where a separator is unnecessary (that is, in this case, the electrolyte itself can function as a separator).

The wound electrode body 80 having such a configuration is accommodated in the case main body 52, and an appropriate nonaqueous electrolyte is placed (injected) in the case main body 52, whereby the wound electrode body 80 is impregnated with the nonaqueous electrolyte. Let As the non-aqueous electrolyte accommodated in the case main body 52 together with the wound electrode body 80, the same non-aqueous electrolyte used in conventional lithium secondary batteries can be used without any particular limitation. Such a nonaqueous electrolytic solution typically has a composition in which a supporting salt is contained in a suitable nonaqueous solvent. As said non-aqueous solvent, ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), propylene carbonate (PC) etc. can be used, for example. Further, as the supporting salt, for _{example, LiPF 6, LiBF 4, LiAsF} 6, LiCF 3 SO 3, can be preferably used a lithium salt of LiClO ₄ and the like. For example, a nonaqueous electrolytic solution in which LiPF ₆ as a supporting salt is contained in a mixed solvent containing EC and EMC at a volume ratio of 3: 7 at a concentration of about 1 mol / liter can be preferably used.

  The non-aqueous electrolyte is housed in the case body 52 together with the wound electrode body 80, and the opening of the case body 52 is sealed with the lid body 54, whereby the lithium secondary battery 100 according to this embodiment is constructed (assembled). ) Is completed. In addition, the sealing process of the case main body 52 and the arrangement | positioning (injection) process of electrolyte solution can be performed similarly to the method currently performed by manufacture of the conventional lithium secondary battery. Thereafter, the battery is conditioned (initial charge / discharge). You may perform processes, such as degassing and a quality inspection, as needed.

  Here, in the lithium secondary battery described above, for example, when charging (particularly, high rate charging) is performed in a low temperature environment, lithium (Li) may be deposited on the negative electrode surface. Since most of the lithium deposited on the negative electrode surface cannot contribute to the charge / discharge reaction of the battery, there may be inconveniences such as a decrease in battery capacity during repeated charge / discharge.

  Therefore, as one of the items for detecting the lithium deposition state at the negative electrode, the present inventor considers a method using impedance measurement for measuring the impedance between the positive and negative electrodes of the battery. In the impedance measurement, for example, an AC signal (typically an AC current or an AC voltage) is applied to the battery while changing the frequency from 0.1 Hz to 10 kHz, and the impedance is measured from the voltage / current response signal. At this time, a plurality of impedances are obtained due to the difference in frequency. Based on such a plurality of impedances, the horizontal part X of the plane coordinate is the real part (Re | Z |) (Ω) of the complex impedance, and the imaginary part (Im | Z |) (Ω) of the complex impedance is shown on the vertical axis Y. By plotting, a Cole-Cole plot as shown in FIG. 3 can be obtained.

In this Cole-Cole plot, the plot obtained when a high-frequency signal is applied is plotted with the lower value of the real part (Re | Z |) (Ω). Further, the plot obtained when the low frequency signal is applied is plotted with the higher value of the real part (Re | Z |) (Ω). In this Cole-Cole plot, two impedance circles (arc components) C _H and C _L usually appear. Large impedance circle C _L appearing in the low frequency side is caused by the resistance and capacitance of the interface between the electrolyte and electrode. On the other hand, a small impedance circle C _H appearing on the high frequency side is due to the reaction resistance and the capacitance of the electrode. Small impedance circle C _H consuming high-frequency side, for example, may be analyzed by fitting to the equivalent circuit shown in FIG. In the equivalent circuit shown in FIG. 4, R0 represents a direct current resistance, R1 represents a reaction resistance, and C1 represents a double layer capacitance. When metallic lithium on the negative electrode is deposited, since the accompanying reaction resistance of the negative electrode it increases, also increases the value of the reaction resistance R1 obtained by the analysis of the high-frequency side of the impedance circle C _H. In other words, the value of reaction resistance R1 obtained by the analysis of the high-frequency side of the impedance circle C _H, it is possible to detect the state of precipitation of lithium at the negative electrode (cell deterioration state).

Here in view of the detection of the lithium deposition, the value of the reaction resistance R1 obtained in the analysis of the impedance circle C _H is, it is desirable to vary significantly before and after deposition of lithium. As a result of various experiments, the present inventor has found that the change in the reaction resistance value before and after the lithium deposition becomes extremely large when impedance measurement is performed with the battery discharged to some extent. Specifically, a plurality of batteries adjusted to different SOCs were prepared, and each battery was subjected to impedance measurement between positive and negative electrodes before and after a cycle deterioration test involving lithium deposition. Among these, the results of impedance measurement for the batteries adjusted to SOC of 0%, 20%, 40%, 60%, and 80% are shown in FIGS. FIG. 8 shows the Cole-Cole plot obtained with SOC 0%, FIG. 9 shows the Cole-Cole plot obtained with SOC 20%, FIG. 10 shows the Cole-Cole plot obtained with SOC 40%, and FIG. FIG. 12 shows a Cole-Cole plot, and FIG. 12 shows a Cole-Cole plot obtained at 80% SOC. FIG. 13 is a graph showing the relationship between the value of the reaction resistance R1 obtained from each Cole-Cole plot and the SOC. Such reaction resistance values are obtained by equivalent circuit fitting shown in FIG.

  In the batteries in which the SOC was adjusted to 20%, 40%, 60%, and 80%, as shown in FIGS. 9 to 13, the reaction resistance value did not change much before and after lithium deposition. On the other hand, in the battery in which the SOC was adjusted to 0%, there was a significant difference in the reaction resistance value before and after lithium deposition. Specifically, the reaction resistance value after lithium deposition increased nearly three times compared with that before the deposition. This means that the detectability of lithium deposition is greatly improved when the SOC is 0%. That is, by using the reaction resistance value obtained with such SOC 0%, the detection accuracy of lithium deposition is improved, and it is possible to appropriately detect the lithium deposition state (battery deterioration state) at the negative electrode. .

  From the above knowledge, the battery detection method according to the present embodiment includes a discharge process, a measurement process, and a state detection process. The discharging step is a step of discharging the lithium secondary battery to be inspected to SOC 10% or less (preferably 5% or less, particularly preferably 0%).

A measurement process is a process of measuring the impedance of the battery discharged by the said discharge process. In this embodiment, as shown in FIG. 4, the impedance is measured at a plurality of frequencies. Then, the real part of the impedance measured at a plurality of frequencies (Re | Z |) and the imaginary part (Re | Z |) was plotted on plane coordinates, the impedance circle resulting frequency side C _H of the reaction resistance R1 Calculate the value. Calculation of such a reaction resistance, for example, the impedance circle C _H of the high-frequency side can be carried out by fitting the equivalent circuit shown in FIG. Alternatively, the value of the reaction resistance R1 may be directly read from the intersection (that is, the point where the imaginary part is zero) between the high-frequency impedance circle _CH and the X axis.

  The state detection step is a step of detecting the state of the battery based on the measured impedance value obtained in the measurement step. In this embodiment, the reaction resistance value calculated in the measurement step is compared with a predetermined threshold value (determination criterion for deterioration) to determine whether or not the battery has deteriorated. For example, when the reaction resistance value calculated in the measurement step exceeds the threshold value, it may be determined that the battery has deteriorated. At this time, by preparing a plurality of determination criteria (threshold values) for the deterioration, the degree of deterioration of the battery can be determined stepwise. In a preferred embodiment, the threshold value (degradation criterion) is an initial reaction resistance value of the battery (that is, a reaction in an initial state in which the battery has not deteriorated (typically before use in a normal use mode)). It is set to a value that is at least twice as large as the resistance value. For example, when the initial reaction resistance value of the battery is 4Ω, the threshold value is generally 8Ω or more, preferably 10Ω or more, and particularly preferably 12Ω or more. In this way, by setting a value that is twice or more larger than the initial reaction resistance value as a threshold value, it is possible to prevent erroneous detection and improve detection accuracy of battery deterioration.

  FIG. 5 is a block diagram showing an example of a lithium secondary battery system 1000 including a detection device that embodies the battery detection method described above. As shown in FIG. 5, the lithium secondary battery system 1000 includes an electronic control that controls the operation of the lithium secondary battery 100, a load 110 connected to the lithium secondary battery 100, and the lithium secondary battery 100 connected to the load 110. The apparatus 120 includes an SOC detection unit 130 that detects the SOC of the battery, and an impedance measurement unit 140 that measures the impedance of the battery. Since the lithium secondary battery 100 is the same as that described above, a detailed description thereof will be omitted.

  The load 110 may include a power consumer that consumes the power stored in the lithium secondary battery 100 and / or a power supply device (charger) that can charge the battery 100. The electronic control unit 120 is configured to control the operation of the lithium secondary battery 100 connected to the load 110, and drives and controls the load 110 based on predetermined information. A typical configuration of the electronic control unit 120 includes at least a ROM (Read Only Memory) storing a program for performing such control, a CPU (Central Processing Unit) capable of executing the program, an input / output port, and the like. Is included. Various signals from a voltage sensor, a current sensor, a temperature sensor, and the like (not shown), signals (outputs) from the SOC detection unit 130 and the impedance measurement unit 140, and the like are input to the electronic control unit 120 through an input port. Further, the electronic control device 120 outputs a drive signal to the load 110 (power consuming machine and / or power supplying machine) through the output port.

  The SOC detection unit 130 is configured to detect the SOC of the lithium secondary battery 100. The SOC detection unit 130 can be arbitrarily selected from those conventionally used in general SOC detection devices. For example, the SOC detection unit 130 can convert a terminal voltage detected by a voltage sensor (not shown) into an SOC. Such conversion can be performed using a map showing the correlation between the terminal voltage and the SOC. You may perform correction | amendment which considered temperature etc. as needed. Alternatively, the SOC detection unit 130 may calculate the SOC by integrating the current detected by a current sensor (not shown). The SOC detection result detected by the SOC detection unit 130 is sent to the electronic control unit 120 as an output of the SOC detection unit 130. The electronic control unit 120 constantly monitors the SOC detected by the SOC detection unit 130, and performs impedance measurement when the detected SOC reaches a predetermined value of 10% or less (for example, 5%, preferably 0%). A command is issued to the impedance measurement unit 140 to start.

The impedance measuring unit 140 is configured to measure the impedance of the lithium secondary battery 100. The impedance measuring unit 140 can be arbitrarily selected from those conventionally used as a general impedance measuring device. For example, the impedance measuring unit 140 may measure the impedance of the battery at a plurality of frequencies by the AC impedance method. The measurement method in the AC impedance method is not particularly limited. For example, an analog method such as a Lissajous method or an AC bridge method, a digital method such as a digital Fourier integration method, or a fast Fourier transform method using noise can be appropriately employed. Impedance plurality of frequency measured in the measuring section 140 may be in a range capable of calculating the value of the reaction resistance R1 from the impedance circle C _H of the high-frequency side. For example, the plurality of frequencies is typically about 1 MHz to 0.1 Hz, and preferably about 1 kHz to 0.1 Hz. This allows calculating the reaction resistance value with high accuracy from the impedance circle C _H of the high-frequency side. The amplitude of the alternating current applied to the battery is not particularly limited, but is preferably about 1 mV to 10 mV (preferably about 3 mV to 8 mV, for example, about 5 mV). Thereby, accurate measurement with good reproducibility can be performed. The impedance measurement result measured by the impedance measurement unit 140 is sent to the electronic control unit 120 as an output of the impedance measurement unit 140.

  The electronic control device 120 includes a state detection unit (not shown) that detects the state of the battery based on the measured impedance value measured by the impedance measurement unit 140. The impedance measurement result measured by the impedance measurement unit 140 is sent from the impedance measurement unit 140 to the state detection unit. The state detection unit detects the state of the battery based on the impedance measurement result sent from the impedance measurement unit 140.

In this embodiment, the state detection unit first plots the real part (Re | Z |) and imaginary part (Im | Z |) of impedance measured at a plurality of frequencies in plane coordinates, as shown in FIG. A simple Cole-Cole plot. Then, the Cole-Cole plot obtained by fitting the equivalent circuit shown in FIG. 4, calculates a value of reaction resistance R1 from the impedance circle C _H of the high-frequency side. Then, based on the calculated value of the reaction resistance R1, the deterioration degree of the battery is determined. In a preferred mode, whether or not the battery has deteriorated by storing a reaction resistance value, which is a criterion for deterioration, in advance in a storage unit (not shown), and comparing this with a measured result as a threshold value. Determine whether or not. At this time, by preparing a plurality of determination criteria (threshold values) for the deterioration, the degree of deterioration of the battery can be determined stepwise. The recording unit can be configured by a recording medium that can be read by the electronic control unit 120, such as a ROM, an HDD, an optical recording medium, a magnetic recording medium, a magneto-optical recording medium, or a flash memory. The impedance measurement conditions, the equivalent circuit to be fitted, and the like can be changed as appropriate according to the specific configuration of the battery.

  In this way, the SOC of the lithium secondary battery is detected, and the impedance of the battery when the detected SOC is a predetermined value of 10% or less (for example, 5%, preferably 3%, particularly preferably 0%). And the state of the battery (deterioration state of the battery) can be detected based on the measured impedance value.

  The electronic control device 120 may output the determination result determined by the state detection unit to a display unit (not shown). In response to the determination result, the display unit can display, for example, a display prompting battery replacement.

  Further, the electronic control device 120 may include a control unit (not shown) that controls charging / discharging of the battery based on the determination result determined by the state detection unit. The control unit drives and controls the load 110 (power consuming machine and / or power supplying machine) so that the battery state is kept good based on at least the determination result determined by the state detecting unit. For example, based on the determination result determined by the state detector, the maximum input / output allowed for the battery 100 is set, and the load 110 is driven and controlled within the set maximum input / output range. possible. For example, the maximum input / output may be set to be smaller as the degree of deterioration of the battery is larger, and the maximum input / output may be set to be larger as the degree of deterioration of the battery is smaller. According to this configuration, since the maximum allowable input / output of the battery is appropriately set according to the deterioration degree of the battery, it is possible to prevent overload on the battery and to suppress further deterioration of the battery. it can.

  As described above, in the lithium secondary battery system 1000 according to the present embodiment, when the SOC detected by the SOC detection unit 130 is a predetermined value of 10% or less (for example, 5%, preferably 0%), the battery Since the impedance of 100 is measured, the state of the battery can be detected easily and with high accuracy. That is, in the low charge state where the SOC is 10% or less, the measured impedance value greatly changes depending on the presence or absence of lithium deposition on the negative electrode. For this reason, by utilizing the measured impedance value obtained in such a low charge state, it is possible to easily and accurately detect the lithium deposition state (battery degradation state) at the negative electrode.

  The lithium secondary battery system 1000 disclosed herein has performance suitable as a battery mounted on a vehicle. Therefore, according to the present invention, as shown in FIG. 6, a vehicle 1 including any of the lithium secondary battery systems 1000 disclosed herein is provided. In particular, a vehicle (for example, an automobile) including the lithium secondary battery system 1000 as a power source (typically, a power source of a hybrid vehicle or an electric vehicle) is provided.

  Hereinafter, although the test example regarding this invention is demonstrated, it is not intending to limit this invention to what is shown to the following test examples.

<Preparation of positive electrode sheet>
As the positive electrode active material, lithium nickelate (LiNiO ₂ ) powder was used. First, a positive electrode active material powder, acetylene black (AB) as a conductive material, polyvinylidene fluoride (PVDF) and carboxymethyl cellulose (CMC) as a binder, and the mass ratio of these materials is 90: 7: 1: 2. The mixture was mixed in N-methylpyrrolidone (NMP) to prepare a positive electrode active material layer paste. The positive electrode active material layer paste is coated on both sides of a long sheet-like positive electrode current collector (aluminum foil having a thickness of about 20 μm) and dried to form a positive electrode active material layer on both surfaces of the positive electrode current collector. A positive electrode sheet provided with was prepared.

<Preparation of negative electrode sheet>
As the negative electrode active material, graphite powder was used. First, the negative electrode active material powder and polyvinylidene fluoride (PVDF) as a binder were mixed in water so that the mass ratio of these materials was 95: 5 to prepare a negative electrode active material layer paste. The negative electrode active material layer paste is applied to both sides of a long sheet-like negative electrode current collector (copper foil having a thickness of about 15 μm) in a strip shape and dried, whereby a negative electrode active material layer is formed on both surfaces of the negative electrode current collector. A negative electrode sheet provided with was prepared.

<Production of lithium secondary battery>
Next, the positive electrode active material layer of the positive electrode sheet was punched out to 3 cm × 4 cm to produce a positive electrode. Moreover, the negative electrode active material layer of the negative electrode sheet was punched out to 3 cm × 4 cm to produce a negative electrode. An aluminum lead is attached to the positive electrode, a nickel lead is attached to the negative electrode, they are arranged facing each other through a separator (a porous polyethylene sheet is used), and inserted into a laminate bag together with a non-aqueous electrolyte, as shown in FIG. A laminate cell 60 was constructed. In FIG. 7, reference numeral 61 denotes a positive electrode, reference numeral 62 denotes a negative electrode, reference numeral 63 denotes a separator impregnated with an electrolytic solution, and reference numeral 64 denotes a laminate bag. As the non-aqueous electrolyte, LiPF ₆ as a supporting salt is contained in a mixed solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3: 7 at a concentration of about 1 mol / liter. Used. In this way, the lithium secondary battery 100 was assembled. Thereafter, an initial charge / discharge treatment (conditioning) was performed by a conventional method to obtain a test lithium secondary battery.

<Measurement of rated capacity>
The rated capacity of the test lithium secondary battery obtained as described above was measured by the following procedures 1 and 2 at a temperature of 25 ° C. and a voltage range of 3.0 V to 4.1 V.
Procedure 1: After reaching 4.1V by constant current charging at 1C, charge at constant voltage and charge until total charge time is 1 hour 30 minutes.
Procedure 2: Discharge to 3.0 V by 1 C constant current discharge, and after 5 minutes of rest, discharge to 3.0 V again by 1/5 C constant current discharge.
Rated capacity: The discharge capacity in constant current discharge in procedure 2 is defined as the rated capacity. In this test lithium secondary battery, the rated capacity is about 14 mAh.

<SOC adjustment>
After the measurement of the rated capacity, the test lithium secondary battery was adjusted to various SOCs. The SOC adjustment was performed by the following procedures 1 and 2. Here, in order to make the influence of temperature constant, SOC adjustment was performed in a temperature environment of 25 ° C.
Procedure 1: Charge at a constant current from 3 V to 1 C until the voltage corresponds to approximately 0%, 20%, 40%, 60%, 80% of the rated capacity.
Procedure 2: After procedure 1, charge at constant voltage for 30 minutes.
Thereby, the test lithium secondary battery can be adjusted to a predetermined SOC.

<Measurement of initial reaction resistance (R1)>
After the above SOC adjustment, AC impedance measurement was performed at a temperature of 0 ° C. in various SOC batteries, and the initial reaction resistance (R1) value (Ω) was obtained by equivalent circuit fitting of the obtained Cole-Cole plot. Asked. The impedance measurement conditions were a frequency range of 10 kHz to 0.1 Hz and a voltage amplitude of 5 mV.

<Cycle deterioration test>
After measuring the initial reaction resistance value, the lithium secondary battery was subjected to a cycle deterioration test in which high-rate charge / discharge was repeated. Specifically, in a constant temperature bath at 0 ° C., a high-rate pulse charge is performed at 24C for 10 seconds, then a high-rate pulse discharge is performed at 4C for 60 seconds, and then a charge / discharge cycle is stopped 500 times continuously. And repeated. The capacity retention ratio (= “capacity after cycle test / capacity before cycle test” × 100) is calculated from the battery capacity before the cycle deterioration (initial capacity of the lithium secondary battery) and the battery capacity after the cycle deterioration. As a result, it was about 80%. Such a decrease in battery capacity results from the deposition of metallic lithium on the negative electrode. That is, most of the lithium deposited on the negative electrode can no longer contribute to the charge / discharge reaction of the battery, so that the battery capacity is presumed to be reduced.

<Measurement of reaction resistance (R1) after cycle deterioration test>
The lithium secondary battery after the cycle deterioration test was adjusted to SOCs of approximately 0%, 20%, 40%, 60%, and 80% in the same manner as in the above <SOC adjustment>. Then, AC impedance measurement was performed on batteries with different SOCs, and the reaction resistance (R1) value (Ω) after the cycle deterioration test was obtained by equivalent circuit fitting of the obtained Cole-Cole plot. The impedance measurement conditions were the same as the initial reaction resistance measurement described above. The results are shown in Table 1 and FIGS. FIG. 8 shows a Cole-Cole plot obtained with SOC 0%, FIG. 9 shows a Cole-Cole plot obtained with SOC 20%, FIG. 10 shows a Cole-Cole plot obtained with SOC 40%, and FIG. FIG. 12 shows a Cole-Cole plot obtained at 80% SOC, respectively. FIG. 13 is a graph showing the relationship between the initial reaction resistance value, the reaction resistance value after the cycle deterioration test, and the SOC.

  As is clear from Table 1 and FIG. 13, in the batteries in which the SOC was adjusted to 20%, 40%, 60%, and 80%, the reaction resistance value did not change much before and after the cycle deterioration test. In the case of the battery used here, the resistance increase ratio (= reaction resistance value after cycle deterioration test / initial reaction resistance value) remained approximately 1.3 or less (1.07 to 1.26). On the other hand, in the battery in which the SOC was adjusted to 0%, the resistance increase ratio reached 2.96, and the reaction resistance value significantly changed before and after the cycle deterioration test. From this result, it was confirmed that the detectability of cycle deterioration (lithium deposition) was greatly improved when the SOC was 0%. From the viewpoint of improving the detection accuracy, the SOC is suitably 10% or less, preferably 5% or less, and particularly preferably 0%. In terms of the resistance increase ratio, approximately 2 or more is appropriate, preferably 2.5 or more, and particularly preferably 3 or more.

  In the case of the battery tested here, SOC 0% corresponds to a voltage of 3.0V, and SOC 100% corresponds to a voltage of 4.1V. Accordingly, the techniques disclosed herein include the following. That is, a detection method for detecting a state of a lithium secondary battery, a discharge process for discharging the battery to a voltage of 3.35 V (SOC 10%) or less, and a measurement for measuring the impedance of the battery discharged by the discharge process A method for detecting a state of a lithium secondary battery, comprising: a step; and a state detection step of detecting the state of the battery based on a measured value of impedance obtained in the measurement step. In a preferred embodiment, in the discharging step, the battery is discharged to a voltage of 3.0 V (SOC 0%).

  As mentioned above, although this invention was demonstrated in detail, the said embodiment is only an illustration and what changed and modified the above-mentioned specific example is included in the invention disclosed here.

For example, in the embodiment described above, the real part of the impedance measured at a plurality of frequencies (Re | Z |) and the imaginary part (Im | Z |) was plotted on plane coordinates, the reaction from the resulting impedance circle C _H Although the value of resistance R1 is calculated and the state of the battery is detected based on the calculated reaction resistance value, the present invention is not limited to this. For example, the state of the battery may be detected from an electric element (resistance, capacitance, etc.) other than the reaction resistance R1 obtained from the Cole-Cole plot. Alternatively, impedance measurement may be performed only at a specific frequency without sweeping the frequency, and the state of the battery may be detected based on the measurement result.

DESCRIPTION OF SYMBOLS 1 Vehicle 10 Positive electrode sheet 12 Positive electrode collector 14 Positive electrode active material layer 20 Negative electrode sheet 22 Negative electrode collector 24 Negative electrode active material layer 40 Separator sheet 50 Battery case 52 Case body 54 Lid 70 Positive electrode terminal 72 Negative electrode terminal 74 Positive electrode lead terminal 76 Negative electrode lead terminal 80 Winding electrode body 82 Winding core part 100 Lithium secondary battery 120 Electronic control device 130 Detection unit 140 Impedance measurement unit 1000 Lithium secondary battery system

Claims (6)

A detection method for detecting a state of a lithium secondary battery,
A discharging step of discharging the battery to SOC 10% or less;
A measuring step of measuring the impedance of the battery discharged by the discharging step;
A state detection step of detecting the state of the battery based on the measured impedance value obtained in the measurement step.   The lithium secondary battery state detection method according to claim 1, wherein in the measurement step, the impedance is measured at a plurality of frequencies.   3. The lithium secondary battery according to claim 2, wherein in the measuring step, a real part and an imaginary part of the impedance measured at the plurality of frequencies are plotted on a plane coordinate, and a reaction resistance value is calculated from the obtained impedance circle. State detection method.   The state detection step determines whether or not the battery has deteriorated by comparing the reaction resistance value calculated in the measurement step with a predetermined threshold value. Lithium secondary battery state detection method.   The lithium secondary battery state detection method according to claim 4, wherein the threshold value is set to a value that is at least twice as large as an initial reaction resistance value of the battery. The lithium secondary battery state detection method according to any one of claims 1 to 5, wherein in the discharging step, the battery is discharged to SOC 0%.

Images