Lithium supplement electrode of lithium ion battery and lithium ion battery
By embedding active lithium on the graphite substrate to form a lithium-ion battery supplementary electrode, the problem of shortening the cycle life of the lithium-ion battery is solved, and more efficient charging and discharge cycle performance and lower environmental and equipment requirements are achieved.
Patent Information
- Application Number
- CN202420287689.X
- Authority / Receiving Office
- CN · China
- Patent Type
- Utility models(China)
- Current Assignee / Owner
- Filing Date
- 2024-02-07
- Publication Date
- 2025-02-21
- Estimated Expiration
- 2034-02-07
AI Technical Summary
During the charging and discharging cycle, lithium-ion batteries have gradually decayed their capacity and reduced their cycle life.
By embedding active lithium on the graphite substrate, lithium-ion battery lithium-enhancing electrode is formed, and lithium-enhancing is supplemented by using active lithium distributed on the surface of the graphite substrate and active lithium embedded inside the graphite substrate.
It significantly improves the charging and discharging cycle performance of lithium-ion batteries, reduces the requirements for the environment and equipment, and increases the safety and stability of lithium supplementation.
Smart Images
Figure CN222514935U_ABST
Abstract
Description
Technical Field
[0001] The utility model belongs to the field of batteries, in particular to a lithium-ion battery lithium-supplementing electrode and a lithium-ion battery. Background Art
[0002] Lithium-ion battery is a lithium-ion battery that mainly relies on the movement of lithium ions between the positive electrode and the negative electrode to work. During the charge and discharge process, Li+ is intercalated and deintercalated back and forth between the two electrodes: when charging, Li+ is deintercalated from the positive electrode and intercalated into the negative electrode through the electrolyte, and the negative electrode is in a lithium-rich state; the opposite is true during discharge.
[0003] During the charge and discharge cycle of lithium-ion batteries, lithium ions are consumed, causing the capacity of the lithium-ion batteries to gradually decay during the cycle, thereby reducing the charge and discharge cycle life of the lithium-ion batteries. Utility Model Content
[0004] The purpose of the utility model is to provide a lithium-ion battery lithium supplement electrode and a lithium-ion battery, so as to overcome the problem that the cycle life of the existing lithium-ion battery is reduced due to lithium ion consumption.
[0005] The technical solution of the utility model is to provide a lithium-ion battery lithium supplement electrode, which is special in that it comprises an electrode body, which comprises a graphite substrate and active lithium embedded in the surface and / or inside of the graphite substrate.
[0006] The utility model forms a lithium-ion battery lithium replenishing electrode by embedding active lithium on the surface and / or inside of a graphite substrate. Compared with the prior art solution of using metallic lithium directly as a third electrode for lithium replenishment, the lithium replenishing electrode of the utility model has lower requirements on the environment and equipment. In the lithium replenishment stage, the active lithium distributed on the surface of the graphite substrate and the active lithium embedded inside the graphite substrate can participate in the lithium replenishment process, which can significantly improve the charge and discharge cycle performance of the lithium-ion battery.
[0007] Furthermore, the graphite substrate is a graphite sheet or a graphite column. Since the utility model is a lithium replenishing electrode, the electrode only participates in the lithium replenishing process and does not affect the internal resistance of the normal charge and discharge cycle of the battery, so commercially available graphite sheets or graphite columns can be directly selected as the substrate, and the thickness of the graphite sheets or graphite columns is generally not limited.
[0008] In order to further improve the stability of the electrode body and protect the active lithium from oxidation, the utility model can also coat the electrode body with a protective film, which can adopt the existing lithium-ion battery separator, has high ion conductivity and good permeability to lithium ions.
[0009] The utility model also discloses a method for preparing the lithium-ion battery lithium supplement electrode, which is special in that it comprises the following steps:
[0010] The lithium-rich material and the graphite substrate are placed in an electrolyte, and a diaphragm that allows lithium ions to pass through is set between the lithium-rich material and the graphite substrate; the diaphragm described here is a conventional diaphragm in a lithium-ion battery. If a protective film has been coated on the electrode body, the diaphragm in this step can be omitted.
[0011] The external power source causes the lithium-rich material to undergo an oxidation reaction, and lithium ions are released from the lithium-rich material and embedded in the graphite substrate, forming a graphite substrate containing active lithium, which serves as a lithium-supplementing electrode for lithium-ion batteries.
[0012] Furthermore, the lithium-rich material may be metallic lithium, or may be a cathode material commonly used in lithium-ion batteries, such as at least one of lithium iron phosphate, lithium cobalt oxide, and lithium manganese oxide.
[0013] Furthermore, the electrolyte is an electrolyte containing lithium ions.
[0014] The utility model also provides a lithium ion battery, comprising a shell having an open end, an electrode assembly located in the shell, and an upper cover assembly for closing the open end of the shell;
[0015] Its special features are:
[0016] The upper cover assembly is provided with a first polarity terminal and a second polarity terminal.
[0017] The positive and negative tabs of the electrode assembly are electrically connected to the first polarity terminal and the second polarity terminal respectively;
[0018] It also includes the above-mentioned lithium-ion battery lithium-replenishing electrode;
[0019] The lithium-ion battery lithium-replenishing electrode is arranged in the shell, and is used to be electrically connected to the first polarity terminal, so that the lithium-ion battery lithium-replenishing electrode can replenish lithium to the electrode assembly.
[0020] The utility model can control the connection between the lithium replenishing electrode of the lithium ion battery and the first polarity terminal to replenish lithium for the lithium ion battery in a controllable manner, such as replenishing lithium in the formation stage, and replenishing lithium after a set time of the charge and discharge cycle of the lithium ion battery.
[0021] Furthermore, the shell includes a bottom plate and a side plate, and the lithium-ion battery lithium replenishing electrode is arranged between the electrode assembly and the bottom plate. The lithium replenishing electrode is parallel to the end face of the electrode assembly, so that the distance between each circle of electrode plates of the electrode assembly and the lithium replenishing electrode is the same. When the lithium replenishing power supply replenishes the electrode assembly, each circle of electrode plates can be quickly and evenly embedded with lithium.
[0022] Furthermore, the lithium-ion battery lithium supplement electrode is electrically connected to the first polarity terminal through a varistor and a diode. By adjusting the resistance of the varistor, the discharge current is regulated, thereby achieving precise and controllable lithium supplementation to prevent lithium precipitation at the negative electrode due to transitional lithiation; based on the diode, the lithium supplement current is unidirectionally conducted to block the lithium insertion process of the lithium supplement electrode during charging and discharging.
[0023] Furthermore, the upper cover assembly is also provided with a third polarity terminal; the lithium-ion battery lithium replenishing electrode is electrically connected to the third polarity terminal. When lithium replenishment is needed, it is only necessary to electrically connect the third polarity terminal to the first polarity terminal.
[0024] The beneficial effects of the utility model are:
[0025] The utility model forms a lithium-ion battery lithium replenishing electrode by embedding active lithium on a graphite substrate. Compared with the prior art solution of using metallic lithium directly as a third electrode for lithium replenishment, the lithium replenishing electrode of the utility model has lower requirements on the environment and equipment. During the lithium replenishment stage, the active lithium distributed on the surface of the graphite substrate and the active lithium embedded in the graphite substrate can both participate in the lithium replenishment process, which can significantly improve the charge and discharge cycle performance of the lithium-ion battery. BRIEF DESCRIPTION OF THE DRAWINGS
[0026] Figure 1 This is a schematic diagram of the structure of the lithium-ion battery lithium supplement electrode in Example 1;
[0027] Figure 2 is a schematic diagram of the structure of a single cell in Example 2;
[0028] Figure 3 This is a schematic diagram of the explosion structure of a single cell in Example 2;
[0029] Figure 4 Schematic diagram of the external electrical connection structure of a single cell in Example 2;
[0030] The accompanying drawings in the figure are marked as follows:
[0031] 1. Graphite substrate; 2. Lead-out terminal; 3. Shell; 31. Bottom plate; 32. Side plate; 4. Upper cover assembly; 41. First polarity terminal; 42. Second polarity terminal; 5. Insulating lining; 6. Lithium supplement electrode; 7. Varistor; 8. Diode. DETAILED DESCRIPTION
[0032] In order to make the above-mentioned purposes, features and advantages of the utility model more obvious and easy to understand, the specific implementation methods of the utility model are described in detail below in conjunction with the drawings of the specification. Obviously, the described embodiments are part of the embodiments of the utility model, but not all of them. Based on the embodiments of the utility model, all other embodiments obtained by ordinary persons in the field without creative work should fall within the scope of protection of the utility model.
[0033] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.
[0034] In the description of the present invention, it should be noted that the orientations or positional relationships indicated by the terms "top, bottom" etc. are based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention.
[0035] During the charge and discharge process of lithium-ion batteries, lithium ions are embedded and released back and forth between the positive electrode and the negative electrode. Due to changes in the structure of the positive and negative electrode active materials, decomposition of the electrolyte, and the formation and destruction of the solid electrolyte (SEI) film on the surface of the negative electrode active material, lithium ions are inevitably consumed, resulting in the continuous decay of the lithium-ion battery capacity and difficulty in having a longer cycle life.
[0036] In order to overcome such problems, the prior art generally adopts the optimization of positive or negative electrode materials to increase the active lithium in lithium-ion batteries, but such solutions require adjustment of the preparation process of the positive and negative electrodes, which makes the production cost of the entire battery higher. There are also solutions in the prior art that use metallic lithium as the third electrode for lithium supplementation. However, the chemical properties of metallic lithium are very active and easily react with moisture in the air. Therefore, the requirements for the environment (air humidity, oxygen content, etc.) and equipment during the supplementation process are relatively high, which increases the difficulty of the process. At the same time, lithium powder is easy to float in the air, resulting in a high safety risk during the supplementation process. In addition, direct lithium supplementation based on metallic lithium is prone to problems such as excessive supplementation leading to negative electrode interface analysis and even short circuit in the battery.
[0037] The present invention forms a lithium-ion battery lithium supplement electrode by embedding active lithium on a graphite substrate. Compared with the prior art solution of optimizing positive or negative electrode materials, the present invention does not need to adjust the preparation process of the positive and negative electrodes; compared with the prior art solution of using metallic lithium directly as the third electrode for lithium supplement, the present invention has lower requirements on the environment and equipment (especially when the electrode body is coated with a protective film, the active lithium in the lithium supplement electrode can be protected from oxidation, and the stability is better), and there is no floating lithium powder, and the safety is higher. In addition, the present invention can also achieve precise and controllable lithium supplement by controlling the lithium supplement current, and prevent the negative electrode from lithium precipitation due to transitional lithiation.
[0038] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments.
[0039] Example 1
[0040] This embodiment is a lithium-ion battery lithium-replenishing electrode 6 . The main body of the lithium-replenishing electrode 6 is a graphite substrate 1 , which is prepared by embedding active lithium on the surface and / or inside of the graphite substrate 1 .
[0041] like Figure 1 As shown, the shape of the graphite substrate 1 can be set as needed, and usually its shape should be adapted to the shape of the corresponding installation space in the lithium-ion battery, and generally a sheet (also called a plate) or a column (also called a strip or a belt) is selected, and it can be provided with a lead-out terminal 2, which can be electrically connected to the corresponding polarity terminal through the lead-out terminal 2. Specifically, the lead-out terminal 2 can be a copper sheet or copper wire set on the graphite substrate 1.
[0042] The graphite substrate 1 can be made of natural graphite or artificial graphite.
[0043] In this embodiment, active lithium can be embedded into the graphite substrate 1 through the following process:
[0044] The lithium-rich material and the graphite substrate 1 are placed in an electrolyte tank containing electrolyte, and a diaphragm is arranged between the lithium-rich material and the graphite substrate 1. The diaphragm here is a diaphragm commonly used in lithium-ion batteries, so that lithium ions can pass through;
[0045] An external power source is used to electrically connect the lithium-rich material to the graphite substrate 1, and the lithium-rich material undergoes an oxidation reaction. Lithium ions are released from the lithium-rich material, pass through the electrolyte, reach the graphite substrate 1, and are embedded in the graphite substrate 1 to form a graphite substrate 1 containing active lithium, which serves as a lithium-ion battery lithium supplement electrode 6.
[0046] It should be noted that the active lithium mentioned here is metallic lithium with electrochemical activity.
[0047] The lithium-rich material in this embodiment is metallic lithium or a lithium alloy, and may also be a positive electrode material for a lithium-ion battery, such as at least one of lithium iron phosphate, lithium cobalt oxide, and lithium manganese oxide.
[0048] The electrolyte of this embodiment is a lithium-ion battery electrolyte, including an electrolyte salt and a solvent. The type of electrolyte salt is not specifically limited and can be selected according to actual needs. As an example, the electrolyte salt can be selected from lithium hexafluorophosphate LiPF 6 、Lithium tetrafluoroborate LiBF 4 , lithium perchlorate LiCI0 4 The type of solvent is not specifically limited and can be selected according to actual needs. As an example, the solvent can be selected from ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), etc.
[0049] It should be noted that the lithium content of the lithium replenishing electrode 6 should be moderate, neither too much to cause lithium deposition nor too little to fail to meet the lithium replenishing requirement. At the same time, the lithium replenishing demand during the cycle should also be considered.
[0050] It can be specifically determined based on the lithium replenishment demand of the target lithium battery to be replenished and the effective lithium replenishment amount.
[0051] In this embodiment, the lithium supplement electrode 6 is provided to avoid the use of a lithium metal third electrode, and the preparation conditions are safer and milder. Moreover, as the lithium supplement electrode 6, active lithium is supplemented as needed during the cycle process, thereby improving the charge and discharge cycle performance of the battery.
[0052] In order to further improve the stability of the electrode body, the present embodiment may also coat the above-mentioned lithium replenishing electrode (the above-mentioned lithium replenishing electrode without a diaphragm may be defined as the electrode body) with a protective film, and the protective film may adopt an existing lithium-ion battery diaphragm, which has high ionic conductivity and good permeability to lithium ions.
[0053] Example 2
[0054] like Figure 2 and Figure 3As shown, this embodiment is a lithium-ion battery, including a shell 3, an upper cover assembly 4 and an electrode assembly; wherein one end of the shell 3 is open, the upper cover assembly 4 covers the open end of the shell 3, and is sealed and connected to the shell 3, thereby closing the open end of the shell 3, and the upper cover assembly 4 is provided with a first polarity terminal 41 and a second polarity terminal 42; the electrode assembly is mainly composed of a positive electrode sheet, a diaphragm and a negative electrode sheet, the electrode assembly is encapsulated in the shell 3 and the positive and negative electrode ears are electrically connected to the first polarity terminal 41 and the second polarity terminal 42 respectively. The lithium-ion battery of this embodiment also includes the lithium supplement electrode 6 in Example 1. The lithium supplement electrode 6 is placed in the shell 3, and the lead-out terminal 2 extends out of the shell (it should be noted that the lead-out terminal 2 and the shell need to be sealed). The number of lithium supplement electrodes 6 can be one or more, which can be set according to specific lithium supplement needs.
[0055] When lithium replenishment is needed, the lead end 2 of the lithium replenishment electrode 6 is electrically connected to the first polarity terminal 41; because the potential of the lithium replenishment electrode 6 is lower than that of the positive electrode, the active lithium in the lithium replenishment electrode 6 is deintercalated into the positive electrode sheet, and the electrons reach the positive electrode sheet through the external circuit, thereby realizing lithium replenishment of the positive electrode sheet of the battery. After lithium replenishment is completed, the lead end 2 of the lithium replenishment electrode 6 and the first polarity terminal 41 can be disconnected.
[0056] like Figure 4 As shown, in this embodiment, a resistor 7 can be connected in the electrical connection path between the lead-out end 2 of the lithium replenishing electrode 6 and the first polarity terminal 41, and the resistance value of the resistor 7 can be adjusted to adjust the discharge current and precisely control the amount of lithium replenishment.
[0057] A diode 8 may also be connected in the electrical connection path between the lead end 2 of the lithium replenishing electrode 6 and the first polarity terminal 41 to achieve unidirectional conduction of the lithium replenishing current and block the lithium insertion process of the lithium replenishing electrode during charging and discharging.
[0058] In some other embodiments, a third polarity terminal may be provided on the upper cover assembly 4; inside the housing, the lead-out terminal 2 of the lithium replenishing electrode 6 is electrically connected to the portion of the third polarity terminal located inside the housing; when lithium replenishment is required, the portion of the third polarity terminal located outside the housing is electrically connected to the first polarity terminal 41; the active lithium in the lithium replenishing electrode 6 is deintercalated into the positive electrode sheet, and the electrons reach the positive electrode sheet through the external circuit, thereby realizing lithium replenishment of the positive electrode sheet of the battery. After lithium replenishment is completed, the third polarity terminal and the first polarity terminal 41 can be disconnected.
[0059] By inserting a varistor 7 in the electrical connection path between the third polarity terminal and the first polarity terminal 41, the discharge current is adjusted by adjusting the resistance value of the varistor 7, and the amount of lithium replenishment is accurately controlled. A diode 8 is inserted in the electrical connection path between the third polarity terminal and the first polarity terminal 41 to achieve unidirectional conduction of the lithium replenishment current and block the lithium insertion process of the lithium replenishment electrode during the charge and discharge process.
[0060] In this embodiment, the timing of lithium replenishment of the lithium-ion battery is not restricted and can be selected according to actual needs. For example, the lead end 2 of the lithium replenishment electrode 6 can be electrically connected to the first polarity terminal 41 during the battery preparation process (for example, before the formation process and after the formation process) to compensate for the loss of lithium ions on the surface of the negative electrode active material due to the formation of the SEI film. The electrode assembly can also be replenished with lithium according to actual needs during the charging and discharging process, storage process, and repair process of the lithium-ion battery. For example, during the charging and discharging process, storage process, and repair process of the lithium-ion battery, according to the attenuation of the discharge capacity of the lithium-ion battery, the third electrode terminal is electrically connected to the first electrode terminal so that the lithium replenishment electrode 6 replenishes lithium to the positive electrode of the electrode assembly, increasing the number of lithium ions that can migrate between the positive and negative electrodes, thereby effectively reducing the capacity loss of the lithium-ion battery and improving the cycle performance and storage performance of the lithium-ion battery.
[0061] The number of times the lithium ion battery of this embodiment is replenished is not specifically limited and can be selected according to actual needs. For example, during the preparation process of the lithium ion battery, the charging and discharging process, the storage process, the repair process, etc. of the lithium ion battery, lithium replenishment is performed once or multiple times as needed. The amount of lithium replenishment during each lithium replenishment is adjusted according to actual needs, for example, by adjusting the discharge current and other parameters to accurately control. Lithium replenishment can also be performed continuously during the charging and discharging process of the lithium ion battery.
[0062] In this embodiment, the lithium-ion battery housing 3 includes a bottom plate 31 and a side plate 32. The graphite substrate 1 of the lithium supplement electrode 6 is a graphite sheet and is placed between the bottom plate 31 and the electrode assembly in the housing 3. Figure 3 As shown, in this embodiment, an insulating liner 5 is further provided between the bottom plate 31 and the lithium supplement electrode 6 to separate the lithium supplement electrode 6 from the bottom plate 31 and the outer shell.
[0063] In some other embodiments, the lithium replenishing electrode 6 can be placed between the side plate 32 and the electrode assembly. However, relative to the present embodiment, the surface of the lithium replenishing electrode 6 is parallel to the large surface of the electrode assembly. In this case, uneven lithium embedding may occur at the winding tail of the electrode assembly. When the lithium replenishing electrode 6 is placed between the bottom plate 31 and the electrode assembly, the surface of the lithium replenishing electrode 6 is parallel to the end surface of the electrode assembly. Thus, the distance between the surface of the lithium replenishing electrode 6 and each circle of electrode plates of the electrode assembly is the same. When the lithium replenishing electrode 6 is used to replenish lithium to the electrode assembly through an external power supply, each circle of electrode plates can be quickly and evenly embedded with lithium.
[0064] In some other embodiments, the graphite substrate 1 of the lithium supplement electrode 6 may be a graphite shell 3 adapted to the shell 3, and may be sleeved in the shell 3 and placed between the electrode assembly and the shell 3. However, the structure is more complicated than that of this embodiment.
[0065] Example 3
[0066] In this example, four waste 24Ah square-shell lithium iron phosphate batteries produced by the same manufacturer and the same batch were taken, two of which were used as comparison samples, and the other two were treated as lithium supplement samples as follows:
[0067] The lithium supplement electrode 6 in Example 1 is additionally provided between the bottom plate 31 of the housing 3 and the electrode assembly;
[0068] The graphite substrate 1 of the lithium supplement electrode 6 of this embodiment is a graphite plate with a thickness of 2 mm, a length of 50 mm, and a width of 24 mm;
[0069] The lithium-rich material used to prepare the lithium-supplementing electrode 6 is lithium iron phosphate;
[0070] The electrolyte used to prepare the lithium supplement electrode 6 is a 24Ah square shell lithium iron phosphate battery electrolyte;
[0071] The surface of the finally formed lithium-supplementing electrode 6 is golden yellow.
[0072] After the preparation of the lithium-supplemented samples was completed, the initial capacities of the two lithium-supplemented samples and two comparison samples were directly tested at 0.5-1C on the charge and discharge tester; specific data can be found in Table 1.
[0073] Afterwards, the two lithium-replenished samples were recharged with the following process:
[0074] The first polarity terminal 41 is connected to the lead-out terminal 2 of the lithium replenishing electrode 6, and the variable resistor 7 is adjusted to make the discharge current size 1 mA, so as to continuously replenish lithium to the electrode assembly.
[0075] After completing lithium replenishment, perform charge and discharge tests:
[0076] At 25℃±5℃, use a 5V-20A charge and discharge tester to charge at constant current (0.75C) to a voltage of 3.65V, then charge at constant voltage to a current of 1 / 20C, leave for 10 minutes, and then discharge at constant current to a cut-off voltage of 2.5V. This is one cycle. The capacity of the battery after the 100th and 200th cycle discharge is tested. Combined with the initial cycle capacity of the battery, the capacity retention rate of the 100th and 200th charge and discharge cycles is calculated. The specific data can be found in Table 1.
[0077] Table 1
[0078]
[0079] It can be seen from Table 1 that after 100 and 200 charge and discharge cycles, the capacity and capacity retention rate of the lithium-supplemented sample are higher than those of the control sample.
[0080] When the repaired lithium-ion battery has been charged and discharged for a period of time, the capacity decays again, and the first polarity terminal and the lithium replenishment electrode lead-out terminal can be connected to perform multiple lithium replenishments.
Claims
1. A lithium-ion battery lithium supplement electrode, characterized in that: The invention comprises an electrode body, wherein the electrode body comprises a graphite substrate and active lithium embedded in the surface and / or inside of the graphite substrate.
2. The lithium-ion battery lithium supplement electrode according to claim 1, characterized in that: The graphite substrate is a graphite sheet or a graphite column.
3. The lithium-ion battery lithium supplement electrode according to claim 1 or 2, characterized in that: Also included is a protective film coated on the electrode body.
4. A lithium ion battery comprising a housing having an open end, an electrode assembly located in the housing, and an upper cover assembly for closing the open end of the housing; Features: The upper cover assembly is provided with a first polarity terminal and a second polarity terminal. The positive and negative tabs of the electrode assembly are electrically connected to the first polarity terminal and the second polarity terminal respectively; It also includes the lithium-ion battery lithium supplement electrode according to any one of claims 1 to 3; The lithium-ion battery lithium-replenishing electrode is arranged in the shell, and is used to be electrically connected to the first polarity terminal, so that the lithium-ion battery lithium-replenishing electrode replenishes lithium to the electrode assembly.
5. The lithium-ion battery according to claim 4, characterized in that: The shell comprises a bottom plate and a side plate, and the lithium-ion battery lithium supplement electrode is arranged between the electrode assembly and the bottom plate.
6. The lithium ion battery according to claim 4 or 5, characterized in that: The lithium supplement electrode of the lithium ion battery is electrically connected to the first polarity terminal through a varistor and a diode.
7. The lithium-ion battery according to claim 4, characterized in that: The upper cover assembly is also provided with a third polarity terminal; The lithium supplement electrode of the lithium ion battery is electrically connected to the third polarity terminal.