Lithium battery implanted FBG temperature sensor and preparation method and application thereof

Abstract

The invention belongs to the technical field of battery temperature detection, and particularly discloses a lithium battery implanted FBG temperature sensor and a preparation method and application thereof. The implanted FBG temperature sensor in the lithium battery comprises a single-mode optical fiber with an FBG, a capillary quartz tube and an adhesive, the adhesive is arranged at the two ends of the capillary quartz tube and is used for fixing the single-mode optical fiber with the FBG; the single-mode optical fiber with the FBG is arranged in the capillary quartz tube under the action of the adhesive and is ensured not to be in contact with the capillary quartz tube; inert atmosphere is arranged in the capillary quartz tube. According to the invention, a mechanical stress shielding cabin is constructed through an original packaging structure with two fixed ends and a suspended middle part, the structural design directly cuts off a transmission path of mechanical stress to a sensing grating on a physical level, pure temperature signals only related to heat accumulation can be obtained without depending on complex algorithm compensation, and the temperature measurement accuracy is greatly improved. And the measurement accuracy is obviously improved.

Description

Technical Field

[0001] This invention relates to the field of battery temperature detection technology, and in particular to an embedded FBG temperature sensor in a lithium battery, its preparation method, and its application. Background Technology

[0002] In the field of lithium-ion battery safety monitoring and thermal behavior research, accurate acquisition of the battery's internal temperature is a crucial foundation for assessing the risks of heat accumulation, performance degradation, and thermal runaway. Traditional battery surface temperature measurements (such as those using attached thermocouples) often exhibit significant hysteresis due to the thermal resistance of battery packaging materials (such as aluminum-plastic film and steel casing), failing to accurately reflect the thermal state inside the cell (especially the core center). Particularly in high-rate charge-discharge and safety failure mechanism studies, the internal heat generation rate far exceeds the surface heat dissipation rate, and the internal temperature may be 10°C or even higher than the surface temperature.

[0003] Fiber Bragg gratings (FBGs) are considered ideally suited for embedding temperature sensors inside lithium-ion batteries due to their small size, strong electromagnetic interference (EMI) immunity, good corrosion resistance, and intrinsic insulation. However, FBG sensing technology faces a core physical challenge when applied inside batteries: the cross-sensitivity problem.

[0004] The center reflection wavelength shift (ΔλB) of a fiber optic grating is affected by both temperature change (ΔT) and strain (Δε), and the relationship is usually expressed as:

[0005] in, Pe ξ is the effective elastic-optical coefficient of the optical fiber, α is the thermal expansion coefficient, and ξ is the thermo-optical coefficient.

[0006] The environment inside a lithium-ion battery is extremely complex. During the charging and discharging process, lithium insertion and extraction reactions occur on the positive and negative electrodes, causing changes in the lattice volume of the electrodes (for example, the volume of a graphite negative electrode expands by about 10% during lithium insertion, while that of a silicon-based negative electrode is even greater). Macroscopically, this manifests as the periodic expansion and contraction of the cell (i.e., the "breathing effect"). In addition, to ensure the energy density and cycle life of the battery, a huge stacking pressure is usually applied during battery assembly.

[0007] If bare optical fiber (FBG) is directly implanted into the battery, the FBG will simultaneously withstand temperature changes within the battery and significant mechanical compressive / tensile stress. During charging, the electrode expansion compresses the optical fiber, generating axial and radial strain, leading to non-thermally induced wavelength drift. Existing data indicates that the temperature measurement error introduced by this stress can be as high as 5–15°C, severely interfering with the assessment of the battery's true thermal state.

[0008] In existing technologies (such as Chinese patent application CN115628826A), although a double-layer encapsulation structure of quartz tube and metal tube is used to protect the optical fiber, it introduces a metal material (GH3030 alloy), which is absolutely forbidden inside a battery, as the metal conductor can puncture the separator, causing a short circuit inside the battery and leading to catastrophic consequences. Furthermore, the diameter of this existing structure exceeds 1mm, severely damaging the flatness of the battery electrode, leading to lithium plating and localized deactivation. Another approach (such as Chinese patent application CN117477050A) uses multiple sets of fiber optic grating sensors to decouple stress using an algorithm. However, this method, due to volume issues, can negatively impact battery performance when embedding two optical fibers. Additionally, because the stress varies across different areas of the electrode, there will be positional deviations when placing the two fibers. According to fiber optic principles, different stresses will result in different decoupling temperatures, leading to uncontrollable errors in the subsequent algorithmic decoupling.

[0009] Therefore, how to provide a fully insulated, ultra-miniature, passive stress-decoupled lithium battery-embedded FBG temperature sensor is a problem that urgently needs to be solved in this field. Summary of the Invention

[0010] In view of this, the present invention provides an embedded FBG temperature sensor in a lithium battery, its preparation method and application, which solves many shortcomings of the existing methods (including damage to the flatness of the battery electrode, leading to lithium plating and local deactivation, affecting battery performance, and large error).

[0011] To achieve the above objectives, the present invention adopts the following technical solution: An in-cell lithium battery FBG temperature sensor, comprising a single-mode optical fiber with FBG, a capillary quartz tube, and an adhesive. The adhesive is disposed at both ends of the capillary quartz tube for fixing the single-mode optical fiber with FBG. The single-mode optical fiber with FBG is placed inside the capillary quartz tube by the action of adhesive, ensuring that it does not come into contact with the capillary quartz tube. The capillary quartz tube contains an inert atmosphere.

[0012] Preferably, the center wavelength of the FBG in the single-mode fiber with FBG is 1545~1555nm, and the grating length is 5~10mm; The FBG in the single-mode optical fiber with FBG is located at the center of the capillary quartz tube along its length.

[0013] Preferably, the diameter of the single-mode optical fiber with FBG is 120~130μm and the length is greater than the grating region length of the FBG; The capillary quartz tube has an inner diameter of 150~250μm, an outer diameter of 300~400μm, and a length greater than the gate region length of the FBG.

[0014] Preferably, the adhesive comprises epoxy resin adhesive.

[0015] Another object of the present invention is to provide a method for fabricating an FBG temperature sensor implanted in a lithium battery, comprising the following steps: 1) Fabricate FBG on single-mode fiber to obtain single-mode fiber with FBG; 2) Adhesive is applied to both ends of the capillary quartz tube, and the single-mode optical fiber with FBG is fixed with the adhesive so that the single-mode optical fiber with FBG does not come into contact with the capillary quartz tube, thus obtaining an FBG temperature sensor implanted in the lithium battery.

[0016] Another objective of this invention is to provide an application of the prepared lithium battery-embedded FBG temperature sensor in the internal temperature detection of lithium batteries.

[0017] Preferably, the detection method includes the following steps: S1: In the stacking or winding process of lithium-ion batteries, the FBG temperature sensor embedded in the lithium battery is implanted between the positive and negative electrode plates of the battery, and high-temperature resistant tape is used to assist in positioning. The end of the single-mode optical fiber with FBG away from the FBG is led out from the aluminum-plastic film or shell of the battery. S2: One end of a single-mode optical fiber with FBG leading out from the aluminum-plastic film or shell of the battery is connected to an optical fiber demodulator. By detecting the drift of the center wavelength of the FBG, the temperature change inside the battery is inverted, thus realizing the internal temperature detection of the lithium battery. Among them, the capillary quartz tube bears the expansion force of the electrode and the stacking pressure inside the lithium battery, so that the internal FBG only responds to temperature changes, realizing passive decoupling of temperature and mechanical stress.

[0018] As can be seen from the above technical solution, compared with the prior art, the present invention has the following beneficial effects: 1. This invention achieves temperature decoupling of fiber Bragg grating sensors under physical-level stress inside the battery, solving the problem of cross-sensitivity in existing technologies. This invention constructs a mechanical stress shielding chamber by encasing a micro-capillary quartz tube outside the optical fiber and using an original encapsulation structure that is fixed at both ends and suspended in the middle. This structural design directly cuts off the transmission path of mechanical stress to the sensing grating at the physical level, and can obtain a pure temperature signal that is only related to thermal accumulation without relying on complex algorithm compensation, thus significantly improving the accuracy of measurement.

[0019] 2. This invention abandons metal materials, and its fully insulated structure is perfectly adapted to the electrochemical environment of lithium-ion batteries, eliminating the risk of short circuits. Moreover, it can withstand the corrosion of hydrofluoric acid (HF) produced by the decomposition of lithium hexafluorophosphate electrolyte, ensuring the long-term stability of the sensor throughout the entire battery life cycle. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0021] Figure 1 This is a schematic diagram of the internal temperature and voltage data in the control group cycle of the present invention; Figure 2 This is a schematic diagram of the internal temperature and voltage data during the 0.1C charge-discharge cycle of the experimental group of this invention; Figure 3 This is a schematic diagram of the internal temperature and voltage data during the 0.2C charge-discharge cycle of the experimental group of this invention; Figure 4 This is a schematic diagram of the internal temperature and voltage data during the 0.5C charge-discharge cycle of the experimental group of this invention; Figure 5 This is a schematic diagram of the internal temperature and voltage data during the 1.0C charge-discharge cycle of the experimental group of this invention. Detailed Implementation

[0022] The present invention provides an FBG temperature sensor implanted in a lithium battery, the FBG temperature sensor comprising a single-mode optical fiber with FBG, a capillary quartz tube and an adhesive.

[0023] In this invention, the adhesive is disposed at both ends of the capillary quartz tube for fixing a single-mode optical fiber with FBG.

[0024] In this invention, the single-mode optical fiber with FBG is disposed inside the capillary quartz tube by means of an adhesive, ensuring that it does not come into contact with the capillary quartz tube.

[0025] In this invention, the capillary quartz tube is filled with an inert atmosphere, specifically an argon atmosphere.

[0026] In this invention, the center wavelength of the FBG in the single-mode optical fiber with FBG is 1545~1555nm, specifically 1546nm, 1548nm, 1550nm, 1552nm, or 1554nm; the grating length is 5~10mm, specifically 6mm, 7mm, 8mm, or 9mm. The center wavelength of the FBG is designed in the C-band for minimal loss. The grating length is controlled at 5mm. The grating length disclosed in this invention is beneficial for point temperature measurement and reduces the "chirping" phenomenon caused by uneven force at both ends of the grating. Increasing or decreasing the grating length is detrimental to temperature measurement.

[0027] In this invention, the FBG in the single-mode optical fiber with FBG is located at the center along the length of the capillary quartz tube. The capillary quartz tube disclosed in this invention has an extremely high Young's modulus, enabling it to rigidly resist the compressive force of external electrodes without deformation. It also provides insulation against stacking pressures as high as 0.5 MPa to 2.0 MPa inside the battery, thereby protecting the internal optical fiber. Furthermore, the capillary quartz tube is an excellent electrical insulator, preventing short circuits when placed inside the battery. In addition, quartz has extremely strong acid resistance, allowing it to withstand long-term corrosion from lithium battery electrolytes.

[0028] In this invention, the diameter of the single-mode optical fiber with FBG is 120~130μm, specifically 122μm, 124μm, 125μm, 126μm, or 128μm; and the length is greater than the grating length of the FBG.

[0029] In this invention, the inner diameter of the capillary quartz tube is 150~250μm, specifically 160μm, 180μm, 200μm, 200μm, or 240μm; the outer diameter is 300~400μm, specifically 320μm, 340μm, 350μm, 360μm, or 380μm; and the length is greater than the grid region length of the FBG, specifically 10mm.

[0030] In this invention, there is a gap of tens of micrometers between the diameter of the single-mode optical fiber and the inner diameter of the capillary quartz tube. This gap is filled with inert gas, which acts as a natural stress-barrier layer. The optical fiber is in a "free-floating" state within this gap. The outer diameter of the capillary quartz tube is much smaller than that of conventional thermocouples or metal-encapsulated sensors. Between the battery electrodes, this thickness can be accommodated by the flexibility of the separator and electrode coating, without causing severe stress concentration.

[0031] In this invention, the adhesive includes epoxy resin, preferably a high-temperature resistant and electrolyte-resistant epoxy resin, specifically a 353ND optical communication-specific adhesive, which can withstand temperatures up to 120°C inside the battery (in the early stages of thermal runaway). The adhesive is only present at the ends of the capillary tube, penetrating into the tube to a depth of 0.5mm to 1mm through capillary action. It is crucial that the adhesive absolutely not flow into the FBG grid region. The FBG grid region must remain exposed (suspended) in an inert gas environment. If the adhesive coats the grid region, the thermal expansion stress after curing will directly act on the grating, leading to measurement failure.

[0032] The lithium battery-embedded FBG temperature sensor of the present invention has an overall structure (single-mode optical fiber, capillary quartz tube, adhesive) that contains no metal materials and requires no additional PEEK insulating sleeve, thus minimizing its size.

[0033] This invention also provides a method for fabricating an embedded FBG temperature sensor in a lithium battery, comprising the following steps: 1) Fabricate FBG on single-mode fiber to obtain single-mode fiber with FBG; 2) Adhesive is applied to both ends of the capillary quartz tube, and the single-mode optical fiber with FBG is fixed with the adhesive so that the single-mode optical fiber with FBG does not come into contact with the capillary quartz tube, thus obtaining an FBG temperature sensor implanted in the lithium battery.

[0034] In this invention, the method for preparing the FBG includes: firstly, stripping the coating layer of the single-mode fiber, preparing a fiber Bragg grating by means of ultraviolet laser phase masking or femtosecond laser point-by-point writing, and then performing annealing treatment to eliminate residual internal stress and improve stability.

[0035] The present invention also provides an application of the prepared lithium battery-embedded FBG temperature sensor in the internal temperature detection of lithium batteries.

[0036] In this invention, the detection method includes the following steps: S1: In the stacking or winding process of lithium-ion batteries, the FBG temperature sensor embedded in the lithium battery is implanted between the positive and negative electrode plates of the battery, and high-temperature resistant tape is used to assist in positioning. The end of the single-mode optical fiber with FBG away from the FBG is led out from the aluminum-plastic film or shell of the battery. S2: One end of a single-mode optical fiber with FBG leading out from the aluminum-plastic film or shell of the battery is connected to an optical fiber demodulator. By detecting the drift of the center wavelength of the FBG, the temperature change inside the battery is inverted, thus realizing the internal temperature detection of the lithium battery. Among them, the capillary quartz tube bears the expansion force of the electrode and the stacking pressure inside the lithium battery, so that the internal FBG only responds to temperature changes, realizing passive decoupling of temperature and mechanical stress.

[0037] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0038] Example 1

[0039] Step 1: Single-mode fiber preprocessing

[0040] Take a standard 2-meter section of single-mode fiber optic patch cord. Use fiber optic strippers to strip the PVC sheath and aramid tensile layer from the middle section of the fiber, exposing a 5cm length of bare fiber (with coating). Next, use precision strippers to strip a 20mm length of the acrylate coating, exposing the quartz cladding (125μm in diameter). Wipe the bare fiber with anhydrous ethanol and lint-free paper to remove any remaining debris.

[0041] Step 2: Fiber Bragg grating writing

[0042] The prepared bare optical fiber was placed on a femtosecond laser writing platform. A 5mm long FBG was written at the center of the bare optical fiber using a phase mask. After writing, a demodulator was connected to monitor the wavelength, and an annealing treatment was performed at 100℃ for 24 hours to stabilize the grating spectrum.

[0043] Step 3: Inserting the capillary quartz tube

[0044] Under microscope-assisted (entire argon atmosphere) guidance, a custom-designed miniature capillary quartz tube (200 μm inner diameter, 350 μm outer diameter, 10 mm long) is carefully threaded through an optical fiber. Due to the extreme fragility of the fiber, this step requires a precision displacement stage or skilled manual operation. The capillary is moved to the position of the FBG (Fiber Bragg Grating). The location of the FBG can be clearly seen using backlighting or a red light fault locator (usually indicated by faint scattered light or markings). The capillary is adjusted so that the 5 mm long FBG grating area is precisely centered within the 10 mm long capillary.

[0045] Step 4: Dispensing and Curing

[0046] Use a micro-syringe to pick up a very small amount of the prepared epoxy resin adhesive (353ND optical communication special adhesive) and apply it to one end of a capillary quartz tube. Due to capillary action, the adhesive will automatically be drawn into the tube.

[0047] Strictly control the amount of adhesive to prevent it from being drawn too deeply into the FBG in the middle.

[0048] First, apply glue to one end for pre-curing. After the optical fiber is relatively fixed to the tube wall, apply pre-tension to the optical fiber to keep it taut without stretching it. Then, apply glue to fix the other end.

[0049] Place the packaged components into an oven and cure them completely according to the adhesive instructions.

[0050] Step 5: Secondary Protection

[0051] A layer of flexible UV adhesive is applied to the glue-cured areas at both ends of the capillary tube and the root of the extended optical fiber to eliminate stress concentration and prevent the optical fiber from breaking due to bending at the tube opening; thus obtaining an FBG temperature sensor implanted in a lithium battery.

[0052] A method for monitoring the internal temperature of a lithium-ion battery includes the following steps: Step S1: In the stacking or winding process of lithium-ion batteries, the FBG temperature sensor is implanted into a predetermined position between the positive and negative electrode sheets, and high-temperature resistant tape is used for positioning assistance. To obtain the most representative temperature, the sensor should be implanted at the geometric center or hot spot area of ​​the cell. For stacked batteries, the sensor is placed in the separator layer between the positive and negative electrode sheets. To avoid affecting the electrochemical reaction, the sensor is placed on the edge area of ​​the tab side, or a very small area of ​​active material is sacrificed and placed in the center. For wound batteries, the sensor is placed in the gap of the innermost winding needle or in the middle layer of the winding (in this embodiment, it is placed in a wound soft-pack battery, with lithium iron phosphate as the positive electrode and graphite as the negative electrode; the specific placement position of the optical fiber is at the geometric center of the winding core).

[0053] Step S2: At the location where the FBG temperature sensor fiber optic cable exits the battery's aluminum-plastic film / casing, perform heat pressing followed by UV adhesive sealing to prevent electrolyte leakage; attach a short section of heat-shrink tubing with hot melt adhesive to the location where the fiber optic cable passes through the sealing area. During vacuum sealing, the hot melt adhesive melts to fill gaps, ensuring the battery's airtightness meets standards; use high-temperature tape (polyimide PI tape) resistant to electrolyte immersion to attach both ends of the capillary quartz tube (i.e., the sealing area) to the separator or electrode blank area. Note that the tape should not be directly applied to the suspended section in the middle of the capillary tube to avoid the tape's shrinkage force compressing the tube body. Arrange the pigtail along the direction of the tab exit. The location where the fiber optic cable passes through the top sealing edge of the aluminum-plastic film is the weakest point in the seal. The fiber optic cable is very thin; if directly heat-sealed, the PP layer of the aluminum-plastic film may not be able to completely fill the gaps around the fiber optic cable, leading to leakage.

[0054] Step S3: Connect the fiber optic demodulator and invert the internal temperature change of the battery by detecting the drift of the FBG center wavelength. The capillary quartz tube bears the expansion force of the electrodes and the stacking pressure inside the battery, ensuring that the internal FBG only responds to temperature changes, achieving passive decoupling between temperature and mechanical stress. In this embodiment, a fiber optic grating demodulator is connected to acquire wavelength data at a frequency of 1~100Hz.

[0055] When the battery undergoes charge-discharge cycles: the positive and negative electrodes expand, applying radial pressure to the sensor. This pressure is entirely borne by the rigid outer wall of the capillary quartz tube. The tube undergoes a very small amount of elastic deformation (which is negligible due to the extremely high modulus of quartz), while the suspended FBG inside feels no pressure at all.

[0056] The elongation of the electrode along the surface direction is transmitted to the quartz tube through friction. The quartz tube may undergo micro-displacement under the fixation of the tape. However, since the FBG is "fixed at both ends and suspended in the middle" inside the tube, and the optical fiber itself has a certain excess length, the external tensile stress is difficult to be transmitted to the grating area.

[0057] The Joule heat and reaction heat generated inside the battery are transferred through the path from the electrode to the separator to the capillary wall to the inert gas / radiation inside the tube to the FBG. Because the quartz tube wall is very thin (75μm) and the system is in a quasi-thermal equilibrium state, the FBG can respond rapidly to changes in ambient temperature.

[0058] In step S3, the passive decoupling of temperature and mechanical stress means that during the battery charge-discharge cycle, the change trend of the FBG reflection wavelength is only related to the internal heat accumulation and heat diffusion of the battery, and is not directly affected by the volume expansion or contraction stress of the battery electrode caused by charge-discharge.

[0059] To verify the effectiveness of the FBG temperature sensor disclosed in this invention in providing stress isolation, lithium batteries with the FBG temperature sensor implanted were subjected to charge-discharge tests at different rates under normal temperature conditions, including 0.1C, 0.2C, 0.5C, and 1.0C. The comparative experimental setup is as follows: Control group: Temperature sensor without capillary quartz tube protection (single-mode optical fiber with FBG as in Example 1 of this invention) was directly implanted inside the battery; Experimental group: The FBG temperature sensor prepared in Example 1 of this invention was used for the experiment.

[0060] The wavelength changes of two sets of FBGs were recorded in real time, and the temperature responses obtained from their inversion were compared and analyzed. Experimental results show that the two sets of sensors exhibit significantly different temperature response characteristics during charging and discharging, and the capillary quartz tube isolation structure can effectively suppress stress-induced abnormal wavelength drift.

[0061] The battery was subjected to a 0.5C charge-discharge cycle at room temperature (29°C). A schematic diagram of the internal temperature and voltage data during the cycle for the control group is shown below. Figure 1 As shown, through Figure 1It can be seen that the internal temperature should have risen during the discharge phase, but after decoupling the temperature from the collected fiber optic signal, the temperature showed a downward trend, which is inconsistent with the expected temperature change trend. Because the FBG sensor is affected by the coupling of two parameters, temperature and stress, without the protection of the capillary quartz tube, the curve is affected by the coupling of both parameters during the discharge phase, resulting in a downward trend. In addition, although the collected data showed an upward trend during the charging process, the increase was too large, and its value differed greatly from the surface monitoring data, which can also be considered to be affected by the combined influence of temperature and stress.

[0062] The battery was subjected to charge-discharge cycles of 0.1C, 0.2C, 0.5C, and 1.0C at room temperature (25°C). A schematic diagram of the internal temperature and voltage data during the cycles in the experimental group is shown below. Figures 2 to 5 As shown, two FBG temperature sensors with capillary quartz tubes prepared according to this invention are implanted inside the pouch battery, located at the internal center and near the positive electrode tab, respectively. Simultaneously, thermocouples are attached to the surface to collect the surface temperature at the corresponding internal locations. Figure 2 It can be seen that the FBG temperature sensor prepared by this invention exhibits stable internal temperature acquisition during the 0.1C charge-discharge cycle, and the temperature change trend is consistent with the existing trend in both the charging and discharging stages, with more accurate numerical values. To verify the temperature measurement reliability of the FBG temperature sensor prepared by this invention under different charge-discharge rate conditions, tests were conducted at 0.2C, 0.5C, and 1.0C charge-discharge cycles, and the test results are as follows. Figures 3 to 5 As shown in the figure. The results indicate that, under the aforementioned different rate conditions, the FBG temperature sensor prepared in this invention can stably and accurately acquire the internal temperature of the battery in real time, and the obtained temperature change trend is consistent with the existing law of battery temperature change. This demonstrates that the temperature sensor described in this invention effectively eliminates the influence of stress on the wavelength drift of the FBG sensor, reduces the difficulty of measuring single temperature parameters with fiber optic FBG sensors, and makes it more reliable and accurate.

[0063] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0064] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A lithium battery-embedded FBG temperature sensor, characterized in that, The lithium battery-embedded FBG temperature sensor includes a single-mode optical fiber with FBG, a capillary quartz tube, and an adhesive. The adhesive is disposed at both ends of the capillary quartz tube for fixing the single-mode optical fiber with FBG. The single-mode optical fiber with FBG is placed inside the capillary quartz tube by the action of adhesive, ensuring that it does not come into contact with the capillary quartz tube. The capillary quartz tube contains an inert atmosphere.

2. The lithium battery-embedded FBG temperature sensor according to claim 1, characterized in that, The center wavelength of the FBG in the single-mode fiber with FBG is 1545~1555nm, and the grating length is 5~10mm. The FBG in the single-mode optical fiber with FBG is located at the center along the length of the capillary quartz tube.

3. A lithium battery-embedded FBG temperature sensor according to claim 1 or 2, characterized in that, The diameter of the single-mode optical fiber with FBG is 120~130μm, and its length is greater than the grating length of the FBG. The capillary quartz tube has an inner diameter of 150~250μm, an outer diameter of 300~400μm, and a length greater than the gate region length of the FBG.

4. The lithium battery-embedded FBG temperature sensor according to claim 3, characterized in that, The adhesive includes epoxy resin adhesive.

5. A method for fabricating a lithium battery-embedded FBG temperature sensor according to any one of claims 1 to 4, characterized in that, Includes the following steps: 1) Fabricate FBG on single-mode fiber to obtain single-mode fiber with FBG; 2) Adhesive is applied to both ends of the capillary quartz tube, and the single-mode optical fiber with FBG is fixed with the adhesive so that the single-mode optical fiber with FBG does not come into contact with the capillary quartz tube, thus obtaining an FBG temperature sensor implanted in the lithium battery.

6. The application of the lithium battery-embedded FBG temperature sensor prepared by the method described in claim 5 in the internal temperature detection of lithium batteries.

7. The application of the lithium battery-embedded FBG temperature sensor according to claim 6 in the internal temperature detection of lithium batteries, characterized in that, The detection method includes the following steps: S1: In the stacking or winding process of lithium-ion batteries, the lithium battery-embedded FBG temperature sensor is implanted between the positive and negative electrode plates of the battery, and high-temperature resistant tape is used to assist in positioning. The end of the single-mode optical fiber with FBG away from the FBG is led out from the aluminum-plastic film or shell of the battery. S2: One end of a single-mode optical fiber with FBG leading out from the aluminum-plastic film or shell of the battery is connected to an optical fiber demodulator. By detecting the drift of the center wavelength of the FBG, the temperature change inside the battery is inverted, thus realizing the internal temperature detection of the lithium battery. Among them, the capillary quartz tube bears the expansion force of the electrode and the stacking pressure inside the lithium battery, so that the internal FBG only responds to temperature changes, realizing passive decoupling of temperature and mechanical stress.

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