Recovery of Lithium Iron Phosphate by Specific Ultrasonic Cavitation Parameters

Abstract

With the widespread use of lithium iron phosphate batteries in various industries, the amount of waste lithium iron phosphate batteries is also increasing year by year, and if not disposed of in a timely manner, will pollute the environment and waste a lot of metal resources. In the composition of lithium iron phosphate batteries, the cathode has an abundance of elements. The ultrasonic method is a crucial method to recover waste LiFePO₄ batteries. It has the following disadvantages, such as the lack of empirical parameters and suitable research equipment. In order to overcome the inefficiency of the LiFePO₄ recycling method, the airborne bubble dynamical mechanism of ultrasound in the removal of lithium phosphate cathode material was studied by a high-speed photographic observation and Fluent simulation and the disengagement process. Mainly aimed at the parameters such as action time, power, frequency, and action position in the detachment process were optimized. The recovery efficiency of lithium iron phosphate reached 77.7%, and the recovered lithium iron phosphate powder has good electrochemical properties, with the first charge–discharge ratio of up to 145 (mAh)/g. It is shown that the new disengagement process established in this study was adopted for the recovery of waste LiFePO₄.

1. Introduction

In 1997, Padhi et al. first reported that lithium iron phosphate with an olivine structure could be used as the cathode material for lithium-ion batteries, and lithium iron phosphate has a theoretical specific capacity of 170 mAh/g and a 3.5 V charging platform for lithium. Compared with traditional lithium-ion battery materials, it is an ideal material for power lithium batteries because of its wide source of raw materials, outstanding safety performance, good cycling performance, good thermal stability, low cost, and no environmental pollution [1,2,3]. As a general rule, LiBs are made of an anode, a cathode, current collectors, a separator, liquid electrolyte, container, and sealing parts [4,5,6].

Lithium iron phosphate (LiFePO₄) is a green cathode material [7,8,9]. It is used as a cathode material for power lithium-ion batteries, which has a lot of advantages, such as low cost of use, long life, high safety, and environmental friendliness [10,11,12,13]. However, there are many waste batteries and lithium iron phosphate materials from the offcuts of the production process; if they are not properly recycled, it will not only bring to environmental pollution but also cause waste of materials [14,15,16,17].

The treatment processes for used lithium iron phosphate cathode materials include pre-processing, wet recovery, bioleaching, electrochemical recovery, mechanical activation treatment, and ultrasonic method recovery [18,19].

Pyrogenic recovery is made by burning the material. In the combustion of the electrode sheets, organic matter and carbon on the sheet are removed, metals and metal oxides are remained and screened to fine powdered materials [20,21]. This recovery process is comparatively simple and does not need special equipment. This method has obvious drawbacks, mainly the high energy consumption during the removal of binder and carbon black from the cathode material, resulting in low metal recovery and low purity of lithium in the obtained recoveries. In addition, it is prone to environmental pollution. Wet recovery technology uses acid and alkali solutions to precipitate and adsorb the metal ions from the cathode material [22], which is a more mature method and can target the lithium in the waste battery. However, this process cannot recover the whole component of retired lithium iron phosphate, the process is long, and the energy consumption and cost are high. Otherwise, it needs complicated equipment and may cause the loss of lithium precipitation in the precipitation of iron removal, which will reduce the recovery rate of lithium [23,24]. Biological recovery technology has a simple process, is environmentally friendly, has low equipment requirements, and can be selective for lithium elements in the cathode material leaching, but there are also a lot of disadvantages, such as the method has a long recovery cycle, the recovery technology is not mature enough, the leaching conditions are difficult to control, and the recovery rate is not high [14,25,26]. The electrochemical method does not require high-temperature treatment or excessive acid-based solution and can obtain lithium with high purity, but the cost is high, and it is difficult to promote commercially [15].

The ultrasonic method has a short production process, simple operation, low energy consumption, and the activation of ultrasound are beneficial to regenerate new lithium iron phosphate materials, but the recovery efficiency is still low. Guo Yafeng et al. [27] used ultrasound-assisted separation to recover aluminum foil, and 100 mL of the strongly polar solvent DMAc could separate 14.6 g of cathode material (LiCoO₂). The organic solvent could be recovered and reused by distillation with a recovery rate of about 78%. Li Qiang et al. [28] studied the leaching of cobalt and nickel from copper removal residue of waste lithium-ion batteries in an ultrasonic field, and the results showed that the leaching rates of cobalt and nickel reached 90% and 87%, respectively, under the conditions of sulfuric acid concentration of 2 mol/L and ultrasonic power of 300 W. Shen Chengyuan [29] used NMP organic solvent to soak the cathode material and destroy the binder. The separation of the cathode material was achieved by adjusting the ultrasonic action time and temperature.

In order to achieve the purpose of efficient and green recycling of lithium iron phosphate and aluminum foil, this study aims to study the mechanism and technical process of separating lithium iron phosphate from aluminum foil by ultrasonic method. As ultrasonic separation is a dynamic cavitation process [30,31], the combination of high-speed photographic observation and fluent simulation is used to reveal the detachment mechanism of lithium iron phosphate cathode material and find out the influencing factors of the separation.

2. Materials and Methods

2.1. Materials

The materials for this experimental study were sourced from the Shanghai Battery Manufacturing Company (Shanghai, China). The test material is shown in Figure 1.

2.2. Methods

The structure of the positive electrode sheet is aluminum foil in the middle and lithium iron phosphate coating on the front and back. Cavitation of ultrasound is closely related to the cavitation medium temperature, the ultrasound frequency, power, and duration time of action [20,21]. In this study, three research methods were used to explore step-by-step how the cavitation effect can be applied to maximize the benefits of lithium iron phosphate recovery. Firstly, high-speed photographic analysis was used to observe the role of the vacuole population and the location where cavitation occurs. Kinetic and fluid simulation analyses were then used to investigate the factors that influence cavitation. Finally, the important reference factors derived from the three analyses were added to the experiments, and the experimental data were generated.

2.2.1. Mechanism Analysis

High-Speed Photographic Analysis

The ultrasonic cavitation process near the cathode material was recorded with a high-speed camera model i-speed7 from iX Cameras, UK, and slow playback was used to observe the generation and collapse of cavitation bubbles and the separation of lithium iron phosphate from the aluminum foil. In the test, to attain a good observation for the ultrasonic cavitation process, the camera focal length, pixels, horizontal resolution and backlight compensation, etc., were carefully adjusted. The high-speed photography parameters are set as shown in

Table 1. High-speed photography parameters.

Parameters	Value
Resolution	1680 × 1242
Frame speed	10,000/s
Recording time	2.5 s

.

The high-speed photographic analysis is designed to perform a blank test, i.e., without putting in any specimens, the ultrasonic generator is switched on, and the position of the cavitation bubbles produced is observed [32,33]. The high-speed photographic analysis experimental setup consists of three main modules, as shown in Figure 2, namely the ultrasonic cavitation visualization module, the ultrasonic generator module, and the high-speed photographic module. Among them, the ultrasound generation module mainly consists of a piezoelectric transducer and a controller, which converts the high-power signal at the drive power end into mechanical vibration. In order to realize the ultrasonic visualization, a glass cylinder, and transducer collocation structure are designed in this paper. The transducer is a fixed frequency of 40 kHz, and the power is adjustable frequency from 0 to 100 W. During the test, the flash was first switched on, and the flashlight source was aligned to the surface of the ultrasonic transducer, then the ultrasonic generator was switched on, and the shooting was controlled by using a high-speed photography PC terminal.

Ultrasonic Cavitation Dynamics Analysis

Cavitation involves the pressure and duration of bubble generation and collapse. Considering water as a non-viscous fluid, when using water as a medium for ultrasonic cavitation separation, the surface pressure of the bubble caused by cavitation of water can be expressed by the following equation [34].

$$p(R,t)=p_v+p_{g0}{(\frac{R_0}{R})}^{3\gamma }-\frac{2S}{R}$$

(1)

where, p_v is saturated vapor pressure, p_g0 is initial pressure of the gas inside the bubble, R is radius of the vacuole, R₀ is initial size of the vacuole, S is surface tension.

As can be seen from Equation (1), the vacuolar surface pressure increases with the increase in saturated vapor pressure, which increases with the temperature rise. The vacuolar surface pressure decreases with the rise of liquid surface tension, and the surface tension decreases with the increase in temperature. Therefore, to increase the surface pressure of the vacuole, the temperature should be increased appropriately.

According to the cavitation theory, the radial acceleration of inward motion tends to infinity when the radius tends to zero by integrating the solution of the Rayleigh–Priest equation. The numerical integration of this equation can be performed by taking the R(t) radius as a function of time; the collapse time can be calculated by equation [34].

$$\tau =\sqrt{\frac{3}{2}\frac{\rho }{p_{\infty }-p_v}}{\displaystyle {\int }_0^{R_0}\frac{dR}{\sqrt{\frac{R_0^3}{R^3}-1}}}\approx 0.915R_0\frac{\rho }{p_{\infty }-p_v}$$

(2)

where, τ is cavitation bubble collapse time, ρ is Liquid density, p_v is Saturated vapor pressure, p_∞ is Liquid static pressure at infinity from the bulb, R₀ is Initial size of the vacuole.

Numerical Simulation Study

The temperature of ultrasonic propagation medium and the ultrasonic vibration frequency have an essential influence on the ultrasonic cavitation field. To determine the optimum medium temperature and ultrasonic frequency, the ultrasonic cavitation flow field is simulated and analyzed as follows.

Firstly, taking the lithium iron phosphate recovery test system as a reference system, the two-dimensional simulation model is established using Fluent pre-processing software ANSYS-ICEM; the model is shown in Figure 3, which is meshed and geometrically associated. Import the geometry model into Fluent first, then import the UDF model, a dynamic mesh is used to realize the boundary sinusoidal oscillation to simulate the ultrasonic vibrating plate interface [35]. Set the displacement equation of the emotional edge as

$$y=a\sin (2\pi ft)$$

(3)

a is the amplitude and f is the frequency.

Figure 3. ICEM-Simulation mathematical model.

Figure 3. ICEM-Simulation mathematical model.

Set the conditions in the fluent solver. Under general solver settings, select pressure-based, time select transient, and other defaults. Models open the multiphase flow model. In the viscosity model, select the k-£ model, choose standard k-£ model, and choose standard wall functions in the near-wall processing and other defaults. Select liquid water and water vapor in Fluent database and the default values of each physical quantity; set liquid water as the main phase and water vapor as the secondary phase; meanwhile, add cavitation model to the two-phase interaction. Set the boundary conditions, the model except for the pressure inlet boundary with the atmosphere, the rest are wall boundaries. For the wall boundary settings, the default is kept, the total and static pressures are entered as 0, and the turbulence parameter settings specification method are selected as intensity and viscosity ratio, whose values are set to 0.5 and 5, respectively; the water vapor in the volume fractions (volume fraction) option group is specified as 0.

2.2.2. Experimental Design

After the above mechanism analysis, the test device built during high-speed photography analysis was used to conduct specific tests on the test parameters in the following

Table 2. Experimental study parameters.

Parameters	Value
Ultrasonic medium (water) temperature	273 K
	300 K
	327 K
	354 K
	381 K
Ultrasonic action time	5 min
	10 min
	15 min
	20 min
Ultrasonic frequency	20 kHz
	40 kHz
	60 kHz
	80 kHz
Ultrasonic power	40 W
	50 W
	60 W
	70 W
	80 W
	90 W
	100 W

.

In the test, water was used as the ultrasonic medium. The height of the water was higher than the ultrasonic vibrating plate and the cathode material; the cavitation process near the cathode material and the shedding process of lithium iron phosphate powder on the cathode plate was observed by high-speed photographic instrument. After most of the lithium iron phosphate on the specimen is shed, take out the shed specimen, put it into the vacuum drying oven for drying, measure its weight, and calculate the removal efficiency of lithium iron phosphate.

3. Experimental Results and Discussion

3.1. Experimental Results

3.1.1. High-Speed Photographic Analysis

At the beginning of the test, there were no bubbles in the water. Ten seconds later, the cavitation bubbles appeared first in the surrounding edge area of the specimen (Figure 4a). After the bubbles were dense to a certain extent (see Figure 4b), they gradually spread to the middle area of the specimen. In addition, the bubbles expanded rapidly after generation, and when the diameter developed to 1 mm, the collapse occurred on the surface of the specimen. The time from bubble generation to collapse was 9/10,000 s. As the bubble collapsed, the lithium iron phosphate powder was separated from the aluminum foil surface at the collapsed bubble and slowly dispersed into the surrounding water (Figure 4c).

Figure 5a shows the metallographic image of the surface of the specimen after cavitation. It can be observed that there are many circle traces on the surface of the specimen with a diameter of about 17 nm, which is the detachment part of lithium iron phosphate caused by the bubble collapse during the process of cavitation. The above observations indicate that the process of lithium iron phosphate coming off the aluminum foil under the action of ultrasound corresponds to the formation and collapse of bubbles in water, which is a typical process of cavitation effect. Moreover, cavitation occurs first at the weak edge, producing a clear edge effect. This edge effect was visually and clearly observed by a metallurgical microscope with a 400× magnification. Figure 5b,c show the metallographic view of the specimen edge cross-section, and Figure 5c shows the partial magnification of Figure 5c. In comparing Figure 5c with Figure 5a, there were pores and burrs present at the specimen edge, which makes the edge a weak zone. Thus the cavitation effect is more intense and obvious.

3.1.2. Ultrasonic Cavitation Dynamics Analysis

Under the action of atmospheric pressure, the pressure difference p_∞-p_V in the above equation is 1 bar, from which it can be calculated that the collapse time of the cavitation bubble of 1 mm diameter in the above test is 9.15 × 10⁻³ s, which is close to the experimentally measured collapse time of 9/10,000 s.

Since the water temperature, pressure, density, and saturated vapor pressure during cavitation are unchanged, the collapse time of the bubble during cavitation mainly depends on the initial size of the bubble, which is affected by the ultrasonic frequency, and the collapse time of the bubble can be effectively reduced by appropriately increasing ultrasonic frequency.

3.1.3. Numerical Simulation Study

The fluid temperatures are set to 273 K, 300 K, 327 K, 354 K, and 381 K. The temperature contours obtained from the simulation analysis are shown in Figure 6. In which the red part indicates the liquid phase, the blue part means the gas phase, and the middle region is the gas–liquid mixing zone. The calculated results in Figure 6 show that the cavitation area is more significant at the surface temperature of 273 K and 300 K, and the height of the cavitation area can reach 2 cm. When the temperature exceeds 300 K, the vapor content decreases with the increase in temperature, and no prominent cavitation area appears when it comes to 381 K. From the kinetic analysis, it can be seen that the surface pressure of the vacuole increases as the surface tension of the liquid decreases, while the surface tension of water decreases with the increase in temperature. The higher the vacuole surface pressure, the stronger the effect of collapse, and the lithium iron phosphate powder is more easily dislodged from the aluminum foil. To optimize the impact of bubble collapse and maximize the area of the cavitation area, the water temperature of 300 K is appropriate for the ultrasonic cavitation-based lithium iron phosphate removal method.

When taking ultrasonic frequency as a single variable, the effect of ultrasonic vibration frequency on cavitation was simulated, and the obtained gas-phase cloud diagrams for different frequencies were shown in Figure 7 from this gas-phase cloud diagram, it can be seen that the cavitation area is the largest when the frequency is 40 kHz. When the frequency exceeds 40 kHz, the higher the ultrasonic frequency, the more difficult the cavitation occurs. This is due to the decrease in phase time of acoustic expansion as the acoustic frequency increases. As we know, if the cavitation nucleus does not have enough time to grow into a cavitation bubble, the bubble volume, compression phase time, and impact force will be decreased, the effect of cavitation may be weakened or even been disappeared. When lithium iron phosphate is recovered by the ultrasonic method, the frequency should be set at 40 kHz.

In order to determine the optimum parameters for recovery of lithium iron phosphate with the ultrasonic method, the influence of the action time, temperature, and frequency on the shedding efficiency of LiFePO₄ are experimentally examined and the results are shown in Figure 8. The data results obtained from the experiments were basically consistent with the theoretical analysis, and the highest stripping efficiency of 77.7% was obtained at an ultrasonic power of 80 W, frequency of 40 kHz, ultrasonic water temperature of 300 K, and an action time of 15 min.

When the action time reached 15 min, the shedding efficiency achieved 74.41%, and as the action time continued to increase, the shedding efficiency grew slowly. The middle part of the aluminum foil started to show “pinholes”, and the edge part started to break. The experimental shooting results are shown in Figure 9. This is not the result of a single impact of the cavitation bubble on the specimen but the result of repeated impacts of the cavitation bubble, which is a fatigue damage process. Aluminum foil broken on the subsequent regeneration of lithium iron phosphate powder is very unfavorable, so the ultrasonic action should not be too long. The action time of 15 min is appropriate.

When the temperature is 300 K, the removal efficiency reaches the highest value, and the removal efficiency decreases as the temperature increases. The removal efficiency is related to the power of ultrasonic. Although the power increase can cause the cavitation intensity to increase, it does not always raise with the power of ultrasound increase, for the cavitation tends to saturate after the power reaches a specific value. In this test (Figure 8), when the power of ultrasound is 80 W, the removal efficiency arrives at the highest value. As the power of ultrasound exceeds 80 W, increment of the ultrasonic power will produce a large number of useless bubbles, thus causing the scattering attenuation to increase and the cavitation intensity and removal efficiency decreases rapidly.

3.2. Feasibility Analysis of the Direct Reuse of Recycled LiFePO₄ Material

The recovered product at a removal efficiency of 77.7% was selected for electron microscopy scanning analysis, XRD analysis, and regeneration performance testing.

Scanning electron microscopy was used to observe the crystal with the observation magnification of 1000×, 2000×, 5000×, 10,000×, 20,000×, 30,000× and 40,000× as shown in Figure 10. It could be seen that the lithium iron phosphate crystal recovered by ultrasonic cavitation had clear grain boundaries. The particle size of the material is mainly around 1 μm. Combined with EDS analysis, it can be seen that only a small part of the aluminum element is mixed into the actual recovered lithium iron phosphate powder (

Table 3. Elemental composition of recycled products.

Element	Counts
C	0.09%
O	41.45%
Al	0.30%
P	0.10%
Mn	13.44%
Fe	0.20%
Co	10.13%
Ni	25.38%

), which does not affect the electrochemical performance of its regeneration. Of these, the elements iron, cobalt, and nickel are derived from the test raw materials.

The XRD characterization of the pre-recycled lithium iron phosphate material and the post-recycled lithium iron phosphate material was performed using an X-ray diffractometer to compare their spectra as shown in Figure 11, and the results showed that LEP-2 has reduced crystallinity, probably due to reduced Li content, or collapsed Al structure due to cavitation.

To examine the feasibility of regeneration of the recycled lithium iron phosphate powder. The recovered lithium iron phosphate was mixed well with acetylene black, PVDF, in the sample tube according to the mass ratio of 8:1:1, and then an appropriate amount of NMP (1-Methyl-2-pyrrolidinone) solvent was dropped into the above substances until the above-mixed sample became sticky.

The test equipment is Shenzhen Xinwei electrochemical work station. The test environment temperature was (25 ± 1) °C. The Xinwei charge/discharge tester was used to test the charge/discharge performance of the prepared button batteries. The regenerated battery test process is selected as constant current charge and discharge; the constant current charge and discharge are to discharge and charge the assembled secondary battery at a constant current density for testing, and the charge and discharge voltage is set to 3.0 V. Figure 12 shows the cycle performance curve of the assembled button cell at 0.1 C multiplier, from which it can be seen that the first charge/discharge specific capacity of the cell is 145 (mAh)/g and the capacity retention rate is still high after 100 cycles, indicating the good electrochemical performance of the regenerated cell. Commercialization of new LiFePO₄ battery with a capacity of 160 (mAh)/g.

4. Conclusions

In this study, with the aim of recovering lithium iron phosphate batteries, suitable experimental research equipment was firstly built by studying the cavitation mechanism, in which high-speed photography was creatively used as an auxiliary observation. The following conclusions were obtained by combining numerical analysis means.

• Ultrasound-based lithium iron phosphate cathode material from aluminum foil is a typical process of cavitation induced by ultrasound. The cavitation bubbles generated in water can effectively remove lithium iron phosphate cathode material from aluminum foil. The pores and burrs at the edge of the specimen are more likely to cavitation, and the edge area is detached wholly and early.
• Theoretical simulation and test results have shown that the ultrasonic action time should not be too long or too short, otherwise the quality of recovered lithium iron phosphate may decline, or the separation of lithium iron phosphate from the aluminum foil will be uncompleted. The best action time is 15 min.
• The ultrasonic power shall be adjusted to 80 W. Too low will fail to induce cavitation phenomenon generation; too high will produce a large number of useless bubbles, thus increasing the scattering attenuation and reducing the cavitation intensity. The ultrasonic frequency shall be set as 40 kHz. Too low will not induce the cavitation phenomenon to occur, and too high will cause the cavitation bubble to collapse in time to form a large area cavitation phenomenon. During ultrasonic treatment, the temperature of the water should be 300 K. Both too high and too low temperatures can inhibit the cavitation phenomenon.
• Under the optimal physical parameters, the efficiency of lithium iron phosphate cathode material removal in the test can reach 77.7%, and the recovered lithium iron phosphate has good electrochemical performance, such as the first charge/discharge specific capacity can reach 145 (mA.h)/g, high-capacity retention after 100 cycles. It is shown that the recovered lithium iron phosphate by the ultrasound method established in this study can be directly used as the cathode material.