Patent Application
20220376312
The present invention relates to recycling and regenerating lithium-ion batteries using an approach that is non-destructive, effective, energy efficient, environmentally friendly, and amenable to industrial mass production.
With the growing applications of lithium-ion batteries (LIBs) in many areas, their recycling becomes a necessary task. Although great effort has been made on LIB recycling, there remains an urgent need for green and energy-efficient approaches.
LIBs have been widely used in mobile electronics, electric vehicles (EVs) and renewable grids due to their high energy density. Typical LIBs will reach their lifetime after a few years of service due to performance degradation. It is projected that ˜1 million tons of used LIBs will be extracted from the market by 2025. From the economic point of view, reuse of the precious metals (e.g., $90/kg for Co, $14/kg for Ni, $20/kg for Li) from LIBs can significantly reduce their cost because a significant portion (30-40%) of the LIB cost comes from their cathode materials. From the environmental point of view, the flammable and toxic wastes (organic solvents, heavy metals) generated from disposal of used batteries can cause severe environment pollution. Therefore, it becomes strongly desired to recycle, reuse and re-manufacture LIBs for sustainable energy storage.
While less than 5% of used LIBs are recycled today, there has been increasing studies of various recycling technologies. Among different cathode materials, LiCoO2 is the most extensively studied one since it is the first-generation of LIB cathode and has been the dominating cathode material in LIBs for mobile electronics due to its high volumetric and gravimetric energy density. The most common approaches for LiCoO2 recycling are based on chemical leaching followed by electrolysis or chemical precipitation. For example, Zou et al. developed a practical process to recycle various cathode materials including LiCoO2 with high efficiency by pH-controlled precipitation. Corrosive acids are used in such recycling process which requires careful neutralizing treatment to recover the digested metals. Meanwhile, this procedure requires multiple complicated steps to maximize the recovery efficiency and reduce the waste generation. More importantly, the embedded energy in the desired cathode particles is lost during such a destructive recycling process.
Researchers are also trying to develop simple and low-cost recycling approaches. Recently solid-state synthesis method has also been used, in which LiCoO2 harvested from spent LIBs is sintered with a pre-determined amount of Li salt (e.g., Li2 CO3). The synthesis approach is relatively simple, however the Li/Co ratio must be accurately measured before the dosage of Li2 CO3 is determined. The potential limitation of this approach is that the regeneration conditions may differ between each individual cell because the Li/Co ratio changes with the cycling history from cell to cell. An aqueous pulsed discharge plasma approach to renovate LiCoO2 has also been developed, which allows batch processing of spent LIBs. However, the electrochemical performance of renovated LiCoO2 is not ideal since the first discharge capacity is only 126.7 mAh g−1 at C/5 (140 mAh g−1 is often expected for fresh LiCoO2 based on 0.5 Li+ reaction). Similarly, an ultrasonic irradiation approach can only generate recovered LiCoO2 with a first discharge capacity of 131.8 mAh g−1 at C/5. In short, even though great effort has been made to recycle and regenerate LiCoO2 cathode material, an environmental benign approach that both guarantees high electrochemical performance and allows easy processing is still urgently needed.
In addition to the most extensively studied LCO, layered oxide LiNix Coy MnzO2 (0<x,y,z<1, x+y+z=1) (NCM) is becoming the dominating cathode material in the state-of-the-art LIBs due to the high capacity and reduced cost. NCM has degradation issues after cycling due to the Li loss and phase changes. So far, the recycling of NCM cathodes has been mainly based on the hydrometallurgical process. Directly resolving these issues to generate new NCM cathodes can not only reduce the high cost but also prevent environmental pollution from disposal of used LIBs. However, currently there is no effective approach to tackle this challenge. Therefore, there is an urgent need to develop a more energy-efficient, non-destructive process to directly recycle NCM cathodes.
Systems and methods are provided for recycling and regenerating lithium-ion batteries by combining hydrothermal treatment of cycled electrode particles with short thermal annealing to directly regenerate degraded LiCoO2 (or LCO) and LiNix Coy MnzO2 (or NCM) cathode materials. Combining hydrothermal treatment with short thermal annealing to regenerate degraded LiCoO2 particles provides successful reconstruction of stoichiometry composition, desired crystalline structure and superior electrochemical performance from severely degraded cathode materials, and in further embodiments, successful regeneration of degraded NCM cathodes delivers NCM particles with recovered stoichiometry composition, desired crystalline structure and electrochemical performance reaching that of new NCM cathode materials.
In one aspect of the invention, a method for regenerating degraded LiCoO2 (LCO) cathode materials includes pre-dosing lithium (Li) into Li-deficient cathode particles in a Li-containing salt solution; performing a hydrothermal treatment on the salt solution; and thermally annealing the hydrothermally treated salt solution to create regenerated cathode particles.
In another aspect of the invention, a method for regenerating degraded LiNix Coy MnzO2 (NCM) cathode particles includes pre-dosing lithium (Li) into Li-deficient cathode particles in a Li-containing salt solution; exposing the Li-containing salt solution to a hydrothermal treatment; and thermally annealing the hydrothermally treated salt solution to produce regenerated cathode particles.
In still another aspect of the invention, the raw material costs involved in hydrothermal relithiation can be substantially reduced by either replacing the typically employed 4 M LiOH solution by a cost-effective mixture of 0.1 M LiOH and 3.9 M KOH, or recycling of the concentrated 4 M LiOH for continuous relithiation process. For the annealing step, the optimal temperature can be reduced from 850° C. to 750° C. when Li2 CO3 is replaced by LiOH as the Li source to compensate for Li loss at high temperature annealing. Life cycle analysis suggests that this strategy results in a reduced energy consumption and greenhouse gas emissions, leading to an increased potential revenue, particularly when compared with existing hydrometallurgical and pyrometallurgical recycling methods.
In yet another aspect of the invention, a method for regenerating degraded lithium-ion battery cathode material includes hydrothermally treating the cathode material in a Li-containing salt solution at a treatment temperature within a range of about 160° C. to about 220° C. for a treatment period of from 1 to 6 hours; separating the treated cathode material from the salt solution; and annealing the separated cathode material for an annealing period of from 1 to 6 hours to produce a relithiated material. In some embodiments, the cathode material is one or more of LiCoO2, LiMn2O4, LiFePO4, and LixNiy Mnz C1−y−zO2 (0<x,y,z<1). The salt solution may be one or more lithium salt selected from lithium hydroxide (LiOH), lithium carbonate (Li2 CO3), lithium sulfate (Li2SO4), lithium chloride (LiCl), and lithium nitrate (LiNO3). The salt solution may also include one or more of sodium hydroxide (NaOH), potassium hydroxide (KOH), and ammonium hydroxide (NH4OH) and may be a mixture of a lithium salt and KOH to yield approximately 0.1 M to 4 M OH−. In some embodiments, the lithium salt is LiOH and has a concentration of approximately 0.1 M. The Li-containing salt solution may be recycled from at least one prior use.
In some embodiments, the treatment temperature is approximately 220° C., and the treatment period may be approximately 2 to 4 hours. The treatment temperature and treatment period are selected to refill lithium deficiencies in a bulk crystal structure of the cathode material. Annealing may be performed at an annealing temperature within a range of 550° C. to 950° C., and may be performed in at least partial oxygen pressure. The salt solution may be a mixture of LiOH and KOH, and annealing is performed at an annealing temperature of approximately 750° C. Annealing may be performed in an air or oxygen environment at approximately 750° C. to 850° C. Annealing may further comprise mixing the treated cathode materials with an excess amount of a lithium source.
In still another aspect of the invention, a method for regenerating degraded lithium-ion battery cathode material includes refilling lithium deficiencies in a bulk crystal structure of the cathode material by hydrothermally treating the cathode material in a Li-containing salt solution for a treatment period of from 1 to 6 hours at ambient pressure; and annealing the treated cathode material at an annealing temperature for an annealing period of from 1 to 6 hours to produce a relithiated material. In some embodiments, the cathode material is one or more of LiCoO2, LiMn2O4, LiFePO4, and LixNiy Mnz Co1−y−zO2 (0<x,y,z<1). The salt solution may be one or more lithium salt selected from lithium hydroxide (LiOH), lithium carbonate (Li2 CO3), lithium sulfate (Li2SO4), lithium chloride (LiCl), and lithium nitrate (LiNO3). The salt solution may also include one or more of sodium hydroxide (NaOH), potassium hydroxide (KOH), and ammonium hydroxide (NH4OH) and may be a mixture of a lithium salt and KOH to yield approximately 0.1 M to 4 M OH−. The lithium salt may be LiOH with a concentration of approximately 0.1 M. The step of hydrothermally treating the cathode material may include exposing the salt solution to a treatment temperature of approximately 160-220° C. The annealing temperature may be within the range of 550-850° C. and the anneal period may be from 2 to 4 hours. The Li-containing salt solution may be recycled from at least one prior use.
Regenerated LiCoO2 particles from spent lithium-ion batteries (LIBs) retain their original morphology and structure and provide high specific capacity and cycling stability. Importantly, they show much better rate capability than particles regenerated through the solid-state synthesis approach. Unlike the conventional chemical leaching or solid-state synthesis approach, which either requires complicated steps of leaching, precipitation and waste treatment or relies on chemical analysis of Li/Co ratio from cell to cell, this non-destructive approach is much simpler and more environmentally friendly and can easily process batteries with different capacity degradation conditions. The methods demonstrate a greener, simpler and more energy-efficient strategy to recycle and regenerate faded LiCoO2 cathode materials with high electrochemical performance. This approach can be widely used to recycle and regenerate LiCoO2 cathode in a large scale, and can be potentially applied to other types of cathode materials in LIBs and mixed cathode chemistry, providing an important foundation for the sustainable manufacturing of energy materials.
In some embodiments, it was determined that the raw material costs can be substantially reduced by either replacing the typically employed 4 M LiOH solution by a cost-effective mixture of 0.1 M LiOH and 3.9 M KOH, or recycling of the concentrated 4 M LiOH for continuous relithiation process. The life cycle analysis suggests that this strategy results in a reduced energy consumption and greenhouse gas emissions, leading to an increased potential revenue, particularly when compared with hydrometallurgical and pyrometallurgical recycling methods.
In one aspect of the invention, the relithiation solution of 4 M LiOH can be replaced with a more cost-effective mixed solution of 0.1 M KOH and 3.9 M LiOH to achieve the same quality of the recycled cathode materials. Alternatively, the concentrated 4 M LiOH solution can be recycled and reused for relithiation without sacrificing the materials properties. For the annealing step, the optimal temperature can be reduced from 850° C. to 750° C. when Li2 CO3 is replaced by LiOH as the Li source to compensate for Li loss at high temperature annealing, which can effectively reduce energy consumption and CO2 exhaustion in the recycle processing.
and
In an illustrative implementation of the inventive system and method, a simple yet efficient non-destructive cathode recycling approach is provided for generating high-capacity and high-rate active particles using LiCoO2 as the model material. Hydrothermal reaction is widely used in the synthesis of various cathode materials and has the capability of generating particles with high crystallinity and desired stoichiometry. Here, we took the advantage of this process to pre-dose Li into Li-deficient cathode particles without concern about the Li/Co ratio. Then we combined hydrothermal treatment with simple thermal annealing to regenerate LiCoO2 with desired microstructure and composition, which led to outstanding electrochemical performance. Compared with the previous approaches, this strategy shows several major advantages: i) it does not require tedious chemical analysis to determine the amount of Li+ loss, and is compatible with batteries at different capacity fading conditions; ii) it does not require long-time, energy-consuming sintering treatment since Li+ is dosed with the correct stoichiometry during the hydrothermal process; and iii) the regenerated active particles have high capacity and improved rate capability compared with solid-state synthesis approach.
For testing, different forms of cells were evaluated. The first type of cell was commercial LiCoO2 cells. Pouch cells with LiCoO2 as the cathode and graphite as the anode were purchased from MTI Corporation (www.mtixtl.com) (2 Ah, EQ-PL-605060-2 C). The pouch cells were cycled in the voltage range of 3-4.5 V using a LAND battery tester for 200 cycles and discharged to 2 V at C/10 (1 C=150 mA g−1) before disassembly. The cathode strips were harvested from the pouch cells, by thoroughly rinsing with dimethyl carbonate. After drying, the cathode strips were soaked in N-Methyl-2-pyrrolidone (NMP) for 30 min followed by sonication for 20 min. The LiCoO2 powders, binder and carbon black were removed from the aluminum substrates. The obtained suspension was centrifuged at 3500 rpm for 5 min and LiCoO2 powders were precipitated, separated and dried for regeneration. Fresh pouch cells were directly discharged to 2 V at C/10 without any cycling before disassembly and the harvested LiCoO2 material serves as the reference material for comparison.
A second cell type was “home-made” LiCoO2 cells, constructed from pristine LiCoO2 powders (MTI Corporation) to perform cycling and harvesting active cathode particles after capacity fading. To fabricate thick electrodes, LiCoO2 powders were mixed with polyvinylidene fluoride (PVDF), and carbon black (Super P65) in NMP at a mass ratio of 93:4:3 to form homogenous slurries. Then the slurries were cast on aluminum foil using a doctor blade and dried in vacuum at 80° C. for 6 h. Circle electrodes were cut and compressed by rolling mill. The active mass loading was about 28 mg/cm2. 2016-type coin cells were assembled with Li metal disc (thickness 1.1 mm) as anode, 1 M LiPF6 in ethylene carbonate and diethyl carbonate (EC:DEC 1:1 wt.) as the electrolyte, and trilayer membrane (Celgard 2320) as the separator. The cells were cycled in the voltage range of 3-4.5 V to gain >50% capacity loss and then discharged to 2 V at C/10. The following harvesting procedure was the same as stated previously. These materials will be used to validate the regeneration procedure, and also compare the electrochemical performance of the pristine LiCoO2 powder and regenerated cathodes from commercial pouch cells.
A third cell type was home-made LiNi1/3 Co1/3Mn1/3O2 (NCM) and mixed pouch cells. To demonstrate that this approach can be potentially used in mixed cathode chemistry, we built home-made pouch cells from pristine NCM powder (Toda America) and mixed pristine LiCoO2-NCM powders with an active mass ratio of 1:1. The electrodes fabrication procedure, electrolyte and separator are the same as in the home-made LiCoO2 cells. Pouch cells were assembled with Li metal foil (thickness 0.75 mm) as anode, and a typical pouch cell had an electrode area of 20 mm×55 mm. The cells were cycled in the voltage range of 3-4.5V to gain >40% capacity loss and then discharged to 2 V at C/10. The following harvesting procedure was the same as stated previously. These materials will be used to demonstrate the feasibility of this technique to regenerated mixed cathode materials.
Two different cathode regeneration methods were compared in terms of their operation characteristics and electrochemical performance of the regenerated products:
1. Combined hydrothermal treatment and short annealing: For hydrothermal treatment, LiCoO2 powders harvested from cycled cells were loaded into a 100 mL Teflon-lined autoclave filled with 80 mL of 4 M lithium hydroxide (LiOH) solution, or a mixed solution of 1 M LiOH and 1.5 M Li2SO4. In some embodiments, a lithium containing solution can be made by a small concentration (e.g., 0.1 M) of lithium salt (e.g., LiOH, Li2SO4, LiCl, LiNO3), and may also be mixed with an alkaline solution from NaOH, KOH, NH4OH or their mixture. The autoclave was kept at a wide range of temperatures and times, at ambient pressure. The treated LiCoO2 powders were washed thoroughly with deionized water, dried and then annealed at different temperatures with a ramping rate of 5° C./min.
2. Solid-state synthesis: For LiCoO2 regeneration from solid-state synthesis, the compositions of Li and Co of cycled cathode materials were first measured by an inductively coupled plasma optical emission spectrometer (ICP-OES, Perkin Elmer Optima 3000 DV). The harvested cathode powders were mixed with Li2 CO3 with agate mortar and pestle. The amount of Li2 CO3 was calculated to obtain the mixture with a Li/Co ratio of 1.05. The 5% excess amount of Li was added to compensate the Li evaporation during the sintering process. The mixtures were sintered at different temperatures and times with a ramping rate of 5° C./min.
The morphology of LiCoO2 powders was observed by Ultra High-Resolution Scanning Electron Microscope (UHR SEM, FEI XL30). The particle size distribution is analyzed with Nano Measurer. The crystal structure of the powders was examined by X-ray Powder Diffraction (XRD) employing Cu Kα radiation. Raman spectroscopy was recorded using a Renishaw inVia confocal Raman Microscope. Spectra were collected between 300 and 900 cm−1 with a 633-nm laser. The crystal structure of the cycled and regenerated NCM materials were examined by Transmission Electron Microscopy (TEM) (FEI Titan 80-300 kV S/TEM).
To evaluate electrochemical performance, the regenerated and non-treated LiCoO2 powders were mixed with PVDF and Super P65 in NMP at a mass ratio of 8:1:1. The resulted slurries were cast on aluminum foils followed by vacuum drying at 80° C. for 6 h. Circle electrodes were cut and compressed, with controlled active mass loading of about 3 mg/cm2. Cells were fabricated by the same procedure as thicker electrodes mentioned above. Galvanostatic charge-discharge was carried out in the potential range of 3-4.3 V. The electrochemical impedance spectroscopy (EIS) tests were performed at discharged state in the frequency range of 106 Hz to 10−3 Hz with signal amplitude of 10 mV by a Metrohm Autolab potentiostat.
The overall recycling and regeneration procedure are illustrated in
To produce faded cathode materials, commercial pouch cells with LiCoO2 as the cathode active material were cycled in the voltage range of 3-4.5 V to speed up the capacity fading. The resulted cell capacity retention was 74% after 200 cycles, as illustrated by the discharge capacity retention graph in
The following short annealing step can increase the crystallinity and eliminate structure defects. For the solid-state synthesis approach with long-term sintering, the following reactions happen:
where x (0.5<x<1) is the mole number of Li in the cathode. LixCoO2 starts to release oxygen at 220° C. and forms Co3O4, which is later reacted with Li2 CO3 to form LiCoO2 again.
The morphology and particle size of the LiCoO2 powders remained nearly the same after cycling and regeneration under different conditions.
It has been demonstrated that the capacity fading of LiCoO2 is mainly contributed by the Li+ loss and the increases of solid-electrolyte interface (SEI). The Li+ loss of the cycled pouch cell was evidenced by the Inductively Coupled Plasma (ICP) result, showing a Li/Co ratio decreased from 0.99 to 0.80 after 200 cycles. For the regeneration process, the target ratio of Li/Co is 1 and the compositions of the cathode materials regenerated by different processes are listed in Table 1, which lists ICP results of regenerated cathode materials from different conditions.
For the hydrothermal treatment with an approximately 4 mole (M) lithium hydroxide (LiOH) solution, an excess amount of Li source is provided in the aqueous phase, which promotes the lithiation of cycled LiCoO2 to reach targeted stoichiometry without controlling the ratio between Li and Co. One major advantage of this step is that degraded LiCoO2 with any Li/Co ratio can be processed together. The Li/Co ratio remains the same in the regenerated particles after short annealing process. For comparison, in solid-state synthesis, the ratio between Li2 CO3 and the cycled LiCoO2 (Li-deficient) needs to be carefully controlled to reach a desired composition after long-time sintering. Also, it is shown that the final Li/Co ratio obtained after solid-state synthesis decreases as the sintering temperature increases due to Li evaporation during long-term sintering, which is consistent with literature.
Phase transitions also take place in Li+-deficient LixCoO2 during cycling (x is the mole number of Li). According to the phase diagram of LixCoO2,26 for 0.5<x<1, layered LiCoO2 transforms to cubic spinel LiCo2O4, and for x<0.5, spinel Co3O4 is also formed besides LiCo2O4. Another major degradation mechanism of LiCoO2 is that the low Li+ conductivity of the spinel LiCo2O4 and Co3O4 phases increases the polarization which also contributes to the extra capacity loss. The XRD patterns of fresh, cycled and regenerated LiCoO2 materials are compared in
The XRD patterns are further analyzed by Rietveld refinement and the results are displayed in Table 2, which lists the lattice parameters of fresh, cycled and regenerated cathode materials.
The Bragg factor (RB) and weighted profile R-factor (Rwp) are both below 4%. The Li+-deficient cathode after 200 cycles has decreased lattice a and increased c values compared with the fresh cathodes, which are attributed to smaller ionic radius of CO4+ than Co3+, and stronger electrostatic repulsion between the layers due to loss of Li+, respectively.
For the hydrothermal treatment approach, several regeneration conditions were tested: some cycled cathode materials were regenerated only through the hydrothermal treatment for comparison, while some materials were annealed at 700° C. and 800° C. after the hydrothermal treatment to increase the crystallinity. The increased crystallinity is confirmed by the decreased lattice a and c parameters after annealing due to a tighter pack of atoms, as well as the decrease in full width at half maximum (FWHM), as illustrated in
Raman spectra further prove the existence of Co3O4 phase in the cathode after 200 cycles and its conversion to LiCoO2 after regeneration, as illustrated by the spectral charts in
The cycling performance of the regenerated LiCoO2 through both hydrothermal treatment with short annealing and solid-state synthesis is illustrated by the Panel (a) in
The regenerated LiCoO2 from solid-state synthesis under different conditions are also compared in Panel (b) of
More interesting results were found in the evaluation of rate capability. The regenerated LiCoO2 materials with similar cycling stability show different rate performance in Panel (a) of
To investigate the mechanism for the different electrochemical performance of LiCoO2 regenerated under different conditions, EIS measurement was performed on the LiCoO2 at discharged state after 100 cycles at 1 C. Panels (a) and (b) of
The LiCoO2 cathode regenerated by sintering at 850° C. has lower SEI and charge transfer resistances than the cathodes sintered at 750° C. and 950° C. The cathode regenerated by hydrothermal treatment followed by 800° C. annealing has the lowest resistances among all the regenerated cathodes, with a SEI resistance of 16.09Ω and charge transfer resistance of 97.44Ω. The superior rate performance of the hydro 800° C. cathode is attributed to its lowest charge transfer resistance which favors the charge transfer reaction for Li+ intercalation.
The linear part of Nyquist plot in the low frequency range is directly related to Li+ diffusion in electrode, and Li+ diffusion coefficient could be calculated using the following equation.
R is the gas constant, T is the absolute temperature, A is the interface between cathode and electrolyte (A=1.6 cm2), n is the number of electrons involved in reaction (n=1), F is the Faraday constant, C is the concentration of Li+ in the electrode (=ρ/M) based on the molecular weight of LiCoO2 (M) and density (ρ), and σ is the Warburg factor. The Warburg factor can be obtained from the slope of Z′ vs. ω−1/2 plots (co is the angular frequency) in the Warburg region. The results of the Z′ vs. ω−1/2 for regenerated cathodes after 100 cycles, along with the linear fitting curves, are shown in Panel (c) of
The hydrothermal treatment was first carried out using 4 M LiOH solution with a high pH of 14.6. Developing more environmental benign operation is always desired for large-scale industrial processes. Therefore, we replaced the 4 M LiOH solution with a mixed solution containing 1 M LiOH and 1.5 M Li2SO4 to reduce the pH to 12.3. With all the other conditions the same, the cycled LiCoO2 treated with the mixed Li salt solution shows similar cycling stability and rate performance with the LiCoO2 treated with 4 M LiOH solution, as shown in
To further verify the effectiveness of this recycling and regeneration approach, the same hydrothermal treatment with short annealing procedure is applied to recycle LiCoO2 from the home-made cells. To have enough LiCoO2 cathode material for recycling, thick electrode with a high mass loading of 28 mg/cm2 were made. After cycling in the voltage range of 3-4.5 V to gain >50% of capacity fading, the cells were discharged to 2 V at C/10 and disassembled. ICP was performed on the harvested LiCoO2 and the composition was determined to be Li0.59 CoO2. The Li+ loss is higher compared with the commercial cathode cycled after 200 cycles, which is believed to result from the Li+ loss in the Li metal anode due to the SEI formation. It should be noted that due to the processing limitation of the lab-scale coin cells, their resistance was higher than the commercial pouch cells. Therefore, Li+ may not fully get back to the cathode even though the cell was discharged to 2 V. The LiCoO2 regeneration was performed by hydrothermal process with 4 M LiOH solution followed by short annealing at 800° C. for 4 h. The XRD patterns of cycled and regenerated LiCoO2 illustrated in
Considering mixed cathode chemistry is widely used in LIBs, it is ideal that the recycling approach could regenerate mixed cathode material. Mixed LiCoO2 (LCO) and NCM cathode is used as the model material to demonstrate the feasibility of this approach to process mixed cathode material. We assembled two types of cells: 1) pure NCM-based pouch cells and 2) pouch cells using LCO−NCM mixed cathode with an active mass ratio of 1:1. After the pouch cells were cycled in the voltage range of 3-4.5 V until the capacity was decayed more than 40%, those pouch cells were disassembled and the cathode material was harvested. To confirm that the NCM cathode material had suffered from Li+ loss, ICP measurement was performed on pristine and cycled NCM cathode. The composition of the NCM cathode is changed from Li1.005Ni0.331 Co0.341Mn0.330O2 to Li0.796Ni0.326 Co0.342Mn0.329O2, which means the NCM cathode has about 20% Li+ loss after cycling.
Through this further study, one difference we found between NCM and LCO cathode material is their tolerance of underdosing and overdosing of Li. During the short annealing process, the evaporation of Li can lead to a slight underdosing of Li. Under such conditions NCM cathode material has the problem of cation mixing between Li and Ni ions, which means Ni2+ might take the place of Li+, deteriorating the electrochemical performance. Therefore, although the hydrothermal treatment can result in stoichiometric Li concentration in regenerated NCM cathode, a small amount of excess Li source (e.g., 5% Li2 CO3) is added to compensate the Li loss for regenerating LCO and NCM mixed cathode material. Considering the optimum temperature for the solid-state reaction to introduce Li was 850° C. according to experimental results, the short annealing temperature was increased from 800° C. to 850° C. for the processing of mixed cathode material. To prove that LiCoO2 has tolerance for slight overdosing of Li, pristine LiCoO2 was sintered with a small amount of Li source at 850° C. for 4 h. The cycling performance of the slightly overdosed LiCoO2 was similar with the pristine material, as illustrated in
Two batches of cycled mixtures were regenerated by this approach. The first batch was the mixture of degraded cathode materials from cycled LCO pouch cells and cycled self-made NCM pouch cells (denoted as cycled LCO+NCM). The second batch is the degraded cathode material from cycled self-made LCO−NCM mixed pouch cell (denoted as LCO−NCM mixed pouch). The mass ratio between LCO and NCM in these two batches was both 1:1. Pristine mixed LCO and NCM material in a mass ratio of 1:1 serves as the reference for its comparison with the regenerated materials. The voltage-capacity profiles of pristine and regenerated mixed cathode materials illustrated in Panel (a) of
Overall, this cathode regeneration strategy offers significant advantages. The common acid leaching approach requires the usage of corrosive acids, as well as complicated neutralization and precipitation steps to recover metals and reduce waste, while the solid-state synthesis requires chemical analysis and accurate control of Li+ dosage which makes large-scale operation difficult. Compared with both approaches, the hydrothermal approach with short annealing requires neither complicated chemical processing nor tedious elemental analysis to determine the Li+ loss, and can readily process batteries with different conditions of capacity degradation. The alkaline solution after processing cathode material can be reused, since the hydrothermal process will only slightly change the concentration of Li+ in the solution. Considering that the solid particles can be easily separated from the solution, we could recycle and reuse the alkaline solution after processing a batch of cathode material. Extra LiOH can be added to the used alkaline solution to compensate the decreased LiOH concentration, and the adjustment could be easily done by measuring the pH value of the alkaline solution. In addition, only a short period of annealing step is needed, rather than the long-time sintering required in solid-state synthesis approach to allow slow solid-state diffusion. Therefore, the hydrothermal approach both increases the ease of operation and decreases the energy cost for processing.
Besides the NMP dissolution approach used in this study to separate active materials, there are other solvents for PVDF to replace the toxic NMP, such as acetone. In addition, the dissolution process is not the only way to separate active materials, binder and conductive additives. Thermal treatment, for example, could be another choice to separate different components in electrodes. Researchers have successfully used thermal treatment to liberate the electrode particles from the current collectors by a vibrating screening, which can be easily performed in large-scale industry process. The details on the separation methods are not further discussed because the focus of the current study is the regeneration of the active material, rather than the separation of different components in electrodes.
To further understand the energy efficiency of this approach, we have compared the energy consumption of this process with the solid-state synthesis approach. Due to the complexity of the energy consumption calculation regarding various instrument and operation efficiency, we simplified the calculation by considering only the energy consumption required to heat the material and keep it at the desired temperature.
According to the equation:
in which Q as the required energy, t as time, m as mass, Cp as the specific heat capacity, A as the surface area exposed to air, h as the convective heat transfer coefficient, T as the temperature of the material, Tamb as the ambient temperature, we can calculate the energy needed for both processes. The parameter values for the energy consumption calculation are provided in Table 4.
We take the volume ratio between solid material (cathode particles) and water in hydrothermal reactor as 1:1, which is easily achievable in this solid-liquid two phase reaction. Considering the processing of 1 kg LiCoO2, A for the mixture of LiCoO2 and LiOH solution is assumed to be 0.033 m2, and A for the pure LiCoO2 powder is assumed to be 0.021 m2, based on the tap density of 3.5 g/cm2 for LiCoO2. For hydrothermal plus short annealing approach, the energy consumption to heat 1 kg LiCoO2 together with LiOH solution to 220° C. and keep 4 h is calculated to be 1589.4 kJ, and to heat LiCoO2 to 800° C. and keep 4 h is calculated to be 4287.5 kJ. The total energy consumption is 5876.9 kJ. For solid-state synthesis, the energy consumption to heat 1 kg LiCoO2 to 850° C. and keep 12 h is calculated to be 10614.1 kJ. Therefore, the energy consumption of hydrothermal treatment is much less than that of the solid-state synthesis, which means the hydrothermal plus short annealing approach is more energy efficient than the solid-state synthesis approach.
In terms of electrochemical performance, the regenerated active particles from this process can achieve better rate capability than those regenerated through the best condition in solid-state synthesis while offer similarly high capacity and cycling stability. Furthermore, the hydrothermal approach can be used to regenerate mixed cathode materials, which makes it more attractive, considering mixed cathode chemistry is more likely to be used in the LIB industry. Therefore, compared with the state-of-the-art approaches, this work provides a promising strategy to regenerate spent LiCoO2 cathodes with easy processing and low energy consumption without generating additional wastes.
In summary, a simple yet efficient non-destructive approach is disclosed to recycle and regenerate LiCoO2 particles from spent LIBs by combining hydrothermal treatment and short annealing. This approach could fully recover the specific capacity and cycling stability of LiCoO2 without changing the original morphology and size distribution. Compared with the solid-state synthesis approach, the LiCoO2 particles regenerated through hydrothermal approach show improved rate capability, which are attributed to the smaller charge transfer resistance and larger Li+ diffusion coefficient. This strategy represents a simple and energy efficient approach to regenerate spent LiCoO2 cathodes with high electrochemical performance, and can be applied on industrial-scale operation. Furthermore, the inventive regeneration process is applicable to other types of cathodes in LIBs, such as LiMn2O4, LiFePO4 and LixNiy Mnz Co1−y−zO2 (0<x,y,z<1).
The aforementioned work on combining hydrothermal treatment with short thermal annealing to regenerate degraded LiCoO2 (LCO) particles has demonstrated the successful reconstruction of stoichiometry composition and desired crystalline structure from severely degraded LCO cathode materials. However, the cathode reactivity and stability may change dramatically with their original composition and crystal structure. The complex chemistry in layered oxide LiNix Coy MnzO2 (NCM) cathodes can influence the change of crystal structure and local phase after cycling, which further affects the regeneration process. Accordingly, challenges may arise from the different degradation mechanisms of NCM cathode materials compared with simple LCO. More specifically, besides the Li+ loss due to the thickening of solid electrolyte interface (SEI), the crystal structure and microphase change on the particle surface (or sub-surface) is a major reason for the capacity degradation in layered oxide cathodes. For LCO, spinel phases such as Co3O4 and LiCo2O4 can form after degradation, while in the case of NCM, the phase change is more complicated. Due to the Li+ deficiency and migration of Ni2+ between the layers, the rock salt phase (e.g., NiO) will form at the surface besides the common spinel phase. Both phases increase the charge-transfer resistance and reduce the cathode performance. However, reconstructing rock salt phase into Li+ conducting layered structure is challenging due to the thermodynamically unfavorable nature of this reaction.
Despite these challenges, a particle-to-particle approach was developed for successful regeneration of degraded NCM cathodes. Using non-destructive methods, nearly ideal stoichiometry, low cation mixing and high phase purity were achieved in the regenerated NCM particles, which offer high specific capacity, cycling stability and rate capability reaching pristine materials. This work represents a simple yet efficient approach to directly regenerate high-performance NCM cathodes with distinct advantages over traditional hydrometallurgical methods and builds an important foundation for the sustainable manufacturing of energy materials.
The effort focuses on developing non-destructive approaches to directly regenerate degraded NCM cathode particles by resolving their compositional and structural defects. Specifically, a hydrothermal treatment combined with a short annealing was used in controlled atmospheres to regenerate NCM cathode particles. As a comparison, direct solid-state sintering approach was also examined to understand the activity of degraded NCM particles. The reaction mechanism was carefully investigated during different cathode regeneration processes. LiNi1/3 Co1/3Mn1/3O2 (NCM111) and LiNi0.5 Co0.2Mn0.3O2 (NCM523) cathodes were selected as the model materials to study the effect of nickel content on the evolution of particle stoichiometry and microphase. With optimized conditions, both spinel and rock salt phases can be fully converted back to layered phase using these direct regeneration approaches, as confirmed by systematic physicochemical characterizations. The lithium storage capacity and cycling stability of the degraded NCM111 and NCM523 cathode particles can be also recovered to the original levels of the pristine materials.
Dry pouch cells (220 mAh) with NCM523 as the cathode and graphite as the anode were directly purchased from Li-Fun Technology (Xinma Industry Zone, Golden Dragon Road, Tianyuan District, Zhuzhou City, Hunan Province, PRC, 412000). Electrolyte was filled in and the pouch cell was sealed by the vacuum sealer (MTI corporation). The electrolyte (LP40) was 1 M LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) with a weight ratio of 1:1. After formation at C/10 (C=150 mA g−1) for the first cycle, the pouch cells were cycled in the voltage range of 3-4.5 V at 1 C for 200 cycles. Commercial LCO pouch cells were purchased from MTI Corporation (2000 mAh, EQ-PL-605060-2 C). NCM111 pouch cells were assembled as described in previous research, with Li metal foil as the anode and a typical electrode area of 20 mm×55 mm. The NCM111 powder was obtained from Toda America. LCO and NCM111 pouch cells were also cycled in the voltage range of 3-4.5 V to gain >20% capacity loss. All pouch cells were discharged to 2 V before disassembly.
The cathode strips were harvested from the pouch cell, thoroughly rinsed by dimethyl carbonate and soaked in NMP followed by sonication. The active materials, binder and carbon black were removed from the aluminum substrate. The suspension is centrifuged and the active materials were precipitated. The precipitation was washed several times and the active materials were harvested and dried.
Regeneration of cathode materials: The composition of cycled cathode was measured by an Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES, Perkin Elmer Optima 3000 DV). For hydrothermal treatment, cycled cathode materials were added in a 100 mL Teflon liner of an autoclave filled with 80 mL of 4 M lithium hydroxide (LiOH) solution. In some embodiments, a lithium containing solution can be made by a small concentration (e.g., 0.1 M) of lithium salt (e.g., LiOH, Li2SO4, LiCl, LiNO3) with an alkaline solution from NaOH, KOH, NH4OH or their mixture. The autoclave was heated (at ambient) for different periods of time and temperatures. The treated powders were washed with deionized water and sintered with 5% excess amount of Li source (Li2 CO3) in oxygen at 850° C. for 4 h with a ramping rate of 5° C./min. 5% excess amount of Li was added to compensate the Li evaporation during the sintering process. For solid-state sintering, the cathode powders were mixed with Li2 CO3 with agate mortar and pestle. The amount of Li2 CO3 was calculated to make the mixture with a Li/Co ratio of 1.05. The mixtures were sintered at 850° C. for 12 h with a ramping rate of 5° C./min. The sintering process was performed in air and oxygen atmosphere, respectively.
Characterization of regenerated materials: The morphology of the powders was observed by Ultra High Resolution Scanning Electron Microscope (UHR SEM, FEI XL30). The particle size distribution was analyzed with Nano Measurer software. The crystal structure of the powders was examined by X-ray Powder Diffraction (XRD) employing Cu Kα radiation. The crystal structure was also examined by Transmission Electron Microscopy (TEM) (FEI Titan 80-300 kV S/TEM). The XPS measurement was performed with Kratos AXIS Ultra DLD with Al Kα radiation.
Electrochemical characterization: The pristine, cycled and regenerated cathode materials were mixed with PVDF, and Super P65 in NMP at a mass ratio of 8:1:1. Then the slurries were cast on aluminum foil using a doctor blade and dried in vacuum at 80° C. for 6 h. Circle electrodes were cut and compressed by rolling mill. The active mass loading was about 3 mg/cm2. Type-2016 coin cells were assembled with Li metal disc (thickness 1.1 mm) as the anode, 1 M LiPF6 in EC:DEC (1:1 wt) as the electrolyte, and trilayer membrane (Celgard 2320) as the separator. Galvanostatic charge-discharge was carried out using a LAND battery testing system in the potential range of 3-4.3 V at 1 C after C/10 in the initial cycle. The electrochemical impedance spectroscopy (EIS) tests were carried out at discharged state in the frequency range of 106 Hz to 10-2 Hz with a signal amplitude of 10 mV by Metrohm Autolab Potentiostats.
Both commercial and home-made cells were used for the demonstration and performance evaluation. The cell assembly and the materials harvesting for LCO and NCM111 pouch cells followed standard procedures. The assembly of NCM523 pouch cells are described in the Experimental section above, with the same materials harvesting procedures as LCO and NCM111 pouch cells. All pouch cells were cycled in the voltage range of 3-4.5 V at 1 C with capacity degradation of more than 20% over approximately 200 cycles, as shown by the graph of cycling performance of the NCM523 pouch cell in
The degraded cathode particles were obtained and subject to hydrothermal treatment with a short thermal annealing step (denoted as HT-SA), or to a direct solid-state sintering treatment by mixing with Li salt (denoted as SS). The regenerated cathode materials were made into slurries to fabricate new coin cells to evaluate their electrochemical performance for the sake of convenience and consistence, as described in the Experimental section. The scanning electron microscopic (SEM) images of pristine NCM523 particles are shown in Panel (a) of
The particle morphology and size distribution of NCM111 samples were also monitored, as illustrated in
With the above understanding, the first step to regenerate the degraded cathode particles is to re-dose lithium using a hydrothermal-based solution impregnation method.
The compositions of different pristine, degraded and regenerated NCM cathode materials as measured by Inductively-Coupled Plasma (ICP) are listed in Table 5. Compared with their pristine composition, both NCM111 and NCM523 particles had about 22% of Li loss after cycling. With the hydrothermal treatment, these degraded particles can be reconstituted with Li to reach the ideal stoichiometry (˜1.0 Li). A following thermal annealing step needs to be performed to reconstruct their desired microphase and crystallinity and maintain the Li concentration in the particles during the thermal treatment.
In the LCO cathode, only spinel phases (Co3O4 and LiCo2O4) are formed after cycling, while in the cycled NCM cathodes, nanoscale domains of rock salt phase often exist besides the spinel phases. To convert the local rock salt MO (M=Ni, Co, Mn) domains back to layered LiMO2, the following reaction should occur:
MO+0.5Li2 CO3+0.25O2↔LiMO2+0.5 CO2 (6)
This reaction indicates that oxygen partial pressure may be an important factor for the conversion process. Therefore, for comparison, the degraded NCM111 and NCM523 particles were mixed with pre-determined amount of Li+ salts to perform direct sintering to reach a target mole ratio between Li and transition metal ions (1.05:1) in both air and oxygen atmosphere. As shown in Table 5, both of the particles can reach desired overall compositions, indicating their non-sensitivity to O2 partial pressure.
For the HT particles, a short annealing treatment at 850° C. for 4 h was performed in O2 to reconstruct the desired crystallinity of the material. Also, considering the possible Li loss during the annealing which can lead to cation mixing, a small amount of excess Li was added to compensate such a Li loss. Similarly, the particles with this short annealing treatment also reached the target stoichiometry.
It is also critical to investigate the evolution of the microstructure defects.
For all regeneration conditions, the a and c lattice parameters change to higher and lower values, respectively. By comparing HT-SA and SS approaches, it shows that the I003/I104 intensity ratio of the particles regenerated by the former approach is higher than the latter. This indicates smaller Li/Ni cation mixing of the material regenerated by the HT-SA, which is further confirmed by the refinement results in Table 6. It is also noted that the cation mixing of the NCM111-SS-air sample is similar with that of the NCM111-SS-oxygen sample, while the cation mixing of NCM523-SS-air is larger than that of NCM523-SS-oxygen. Since a high nickel content is considered the key factor for the formation of the rock salt phase, it is speculated that the rock salt phase tends to form more easily in cycled NCM523 cathode than that in cycled NCM111 cathode. As oxygen atmosphere is a critical factor that turns the rock salt phase into the layered phase, the added Li source may not effectively react with the rock salt phase when the oxygen partial pressure is low, and the migration of Ni2+ to Li+ sites continues to happen in a Li+ deficient state, which leads to higher cation mixing degree in the NCM523-SS-air. Overall, the HT-SA samples show much smaller Li/Ni mixing, suggesting its advantage of offering a more homogenous lithiation and more effective phase conversion for particle regeneration.
Even though no obvious changes in the morphology and particle size distribution are observed after direct regeneration—as shown in
In addition, it was found that the change in microphase of NCM523 shows higher sensitivity to the oxygen partial pressure than NCM111 in SS regeneration. The SS in air can convert the spinel phase to layered phase in cycled NCM 111 cathode, as illustrated by the HR-TEM and FFT images of cycled NCM111-SS-air sample along the [−1-21] zone axis shown in
To provide further evidence of spinel/rock salt phase in the cycled cathodes and the successful reconstruction of the layered phase after regeneration, x-ray photoelectron spectroscopy (XPS) measurement was performed on NCM111 (as shown in
The electrochemical performance of the NCM111 and NCM523 cathode particles in different conditions were evaluated in the voltage range of 3-4.3 V at 1 C (C=150 mA g−1) after one activation cycle at C/10. For NCM111 in Panel (a), the pristine cathode shows a capacity of 145.1 mAh g−1 in the first cycle at 1 C and 123.8 mAh g−1 after 100 cycles. In Panel (b), the non-treated, cycled cathode shows a capacity of 98.4 mAh g−1 after 100 cycles, which is due to the existence of spinel phase at the surface (see
The rate capability and voltage profiles of the pristine and regenerated cathodes at 5 C are compared in Panels (c), (d), (e) and (f) of
To further understand the rate performance, Electrochemical Impedance Spectroscopy (EIS) measurement was performed on pristine and regenerated NCM523 cathodes after 100 cycles at 1 C.
The superior rate performance of the HT-SA sample is attributed to its lowest Rct which favors the charge-transfer reaction for Li+ intercalation. The linear part of Nyquist plot in the low frequency range is directly related to Li+ diffusion in electrode, and the diffusion coefficient (DLi+) could be calculated by the EIS method using Warburg impedance (Table 7). The NCM523-HT-SA sample has the largest DLi+ of 4.85×10−12 cm2 s−1, and the NCM523-SS-oxygen sample has a larger DL (1.22×10−12 cm2 s−1) than the NCM523-SS-air sample (5.55×10−13 cm2 s−1). The lower Rct and higher DLi+ of the regenerated cathode by the HT-SA approach explain its better rate capability than the cathode regenerated by the SS approach. The remaining rock salt phase and cation mixing on the surface of the regenerated NCM523 in air (Panel (d) of
Overall, degraded NCM particles with different compositional deficiencies and microphase impurities can be effectively regenerated using these direct methods that combine hydrothermal lithiation and short annealing, which leads to ideal stoichiometry, low cation mixing and high phase purity. Panel (g) of
To summarize, the inventive scheme allows the regeneration of chemical composition and microstructure in degraded NCM cathodes using a novel particle-to-particle approach. Hydrothermal treatment with a short annealing provides the regenerated NCM cathode particles with high capacity, long cycling stability and high rate performance that of pristine materials, even with high Ni content. Due to the higher nickel content in NCM523 than NCM111 cathodes, the approach of direct solid-state sintering in air can restore the cycling stability of the latter but not the former. The oxygen partial pressure should be maintained at a high level to effectively convert the rock salt phase impurities to the layered phase in NCM cathodes with high Ni content.
The following examples are directed to further enhancements of the inventive regeneration approach of hydrothermal treatment and annealing. Specifically, these improvements are directed to optimization of efficiency from economic and environmental perspectives, with reduced raw materials costs and energy consumption.
The effects of hydrothermal temperatures and relithiation time on the electrochemical performance of the regenerated materials were examined first. D-NCM111 was relithiated at 160°, 200°, and 220° C. with 4 M LiOH for 1, 2, 4 and 6 h. The results of this test are plotted in
The temperature, time and initial capacity data in Table 8 were then evaluated for underlying trends using robust, polynomial regression as described below.
For two independent variables, the general, fitting polynomial was y=a1x1+a2x2+a12x1x2+b1x1(x1−x2)+b2x2(x1−x2), where x1 represented temperature in ° C., and x2, time in hours. The anxn terms represented the linear contributions to y, and the others represented the contributions of the interactions between time and temperature. The linear terms, a1x1+a2x2, were always present in the polynomials.
A search algorithm for candidate polynomials was implemented in Visual Basic for Applications in Microsoft® Excel®. Robust linear regression was used to minimize the effect of noise in the data and Tukey's iterative, biweight function with a tuning constant of 6 was used to weight the data. The goodness-of-fit, r2, was calculated using the expressions given in equations (7)-(9).
where TSS is the total sum of squares, y is the experimentally observed value, RSS is the residual sum of squares and y is calculated from the fit.
The search process generated many polynomials. Candidate polynomials that represented the data with a value of r2≥0.95 were selected for further consideration. The following criteria were used to further limit the number of polynomials. (1) The polynomial had the fewest number of terms; the fit was overdetermined. (2) The standard error had to be 40% or less than the value of the parameter so that, at the 95% confidence level, the value of the fitting parameter would still be larger than twice of the standard error.
Using the fitting polynomial, the values of the fitting parameters are given in Table 9, providing an r2 greater than 0.99.
The value of a2 is much greater than that of a1, indicating that the hydrothermal process time was the most important factor to obtain high-performing cathode materials. The negative value of b2, at first glance, is somewhat puzzling, and indicates that the interaction of time and temperature negatively affected the initial capacity of these materials. In the time and temperature range studied, the value of b2x2(x1−x2) was always negative and tended to decrease (became more negative) with increasing time at a given temperature. Together, these observations indicated that there was an optimum value beyond which additional hydrothermal time was unlikely to be beneficial.
The samples that showed the highest cycling performance, 160°-6 h, 200° C.-6 h, 220° C.-2 h, were selected for additional characterization. They are denoted as HT-160, HT-200 and HT-220, respectively. Referring to
The lattice parameters were determined via Rietveld refinement of the neutron diffraction pattern, where the evolution of the a and c unit cell parameters were plotted in
The valence states of the transition metals are also sensitive to the concentration of Li+ in cathode materials, which were probed via XPS. The XPS spectra of Co 2p in FIG. 29 C for all the samples were analogous, with two main peaks located at 779.86 eV and 794.99 eV corresponding to Co 2p3/2 and Co 2p1/2, respectively. The absence of a satellite peak around 785 eV indicates that the valence state of Co in all the samples was 3+, which is reasonable given that only 10% of the Li+ was extracted from the D-NMC samples, where only Ni is expected to be oxidized due to its lower redox potential. After chemical dilithiation, the main peak at 854.28 eV related to Ni 2p3/2 of T-NCM111 shifted to a higher binding energy of 854.79 eV (D-NCM111), confirming this trend and indicating that Ni was oxidized to a higher valence state. Although the peak shifted back slightly after relithiation at 160° C., the binding energy was still higher than that of pristine T-NCM111, implying that some Ni3+ still existed in the cathode material. Notably, the peak of Ni 2p3/2 shifted back to the same position as that of the pristine T-NCM111 for the samples treated at 200° C. for 6 h and 220° C. for 2 h, suggesting that the compositional defects were completely repaired under these relithiation conditions.
To better understand the effect of hydrothermal temperature and time on the properties of the final cathode product after direct regeneration, the electrochemical cycling performance of the samples relithiated at different temperatures was evaluated in half cells at a rate of C/10 for 4 cycles followed by 50 cycles at a rate of C/3. It should be mentioned that the relithiated samples were annealed with an excess 5 mol % of Li2 CO3 at 850° C. for 4 h before the electrochemical test for a fair comparison with pristine T-NCM111. The chemical extraction of Li+ reduced the discharge capacity of the first cycle at the C/10 rate from 156.5 mAh/g to 146.1 mAh/g (T-NCM111 vs. D-NCM111). When the rate was increased to C/3, D-NCM111 showed a capacity of 133.9 mAh/g, and a capacity retention of 105.3 mAh/g (78.6%) was observed after 50 cycles. The samples that underwent hydrothermal relithiation at 160° C. followed by annealing improved the initial capacity to 154.3 mAh/g and 149.5 mAh/g at C/10 and C/3, respectively, suggesting that a large portion of Li deficiencies has been recovered. However, after 50 cycles the discharge capacity was reduced to 133.6 mAh/g, which corresponds to a capacity retention of 89.4%. When the relithiation temperature was increased to 200° C. and 220° C., the C/10 discharge capacities were found to be further increased to 156.1 and 156.3 mAh/g, respectively, which are nearly the same as that of pristine T-NCM111 (156.5 mAh/g). Notably, even at the C/3 rate, an initial capacity of 150.4 mAh/g and 150.6 mAh/g, respectively, was achieved, reaching the level of pristine T-NCM111 (150.5 mAh/g). At the end of the 50th cycle, the retained capacities were 140.2 and 140.3 mAh/g for the samples relithiated at 200° C. and 220° C., respectively, both of which were comparable to that of T-NCM111 (140.3 mAh/g). Given this performance, it was concluded that the Li deficiencies of D-NCM111 can only be fully refilled at temperatures higher than 200° C. in the limited time frames examined here. Considering the much longer time needed to achieve full relithiation at 200° C. (6 h) as compared to that of 220° C. (2 h), the optimal hydrothermal relithiation temperature and duration are 220° C. and 2 h.
The effect of the lithium-bearing solution composition used in hydrothermal relithiation was then explored. As described in above, 4 M LiOH was used as the relithiation solution. Considering the high cost of LiOH, a more-dilute LiOH solution would be preferable. In order to exclude the effect of pH, a solution consisting of a mixture of 0.1 M LiOH and 3.9 M KOH (to yield 4 M OH−) was used for relithiation.
The crystal structure of D-NCM111 which was relithiated with the mixed solution was characterized by XRD (
XPS was again performed to probe the valence state of Ni in the above samples (
The concentrations of Li+ and K+ in the cathode particles were further examined by ICP-MS measurement (
The electrochemical performance was then evaluated to understand the effect of the mixed solution on the properties of the regenerated sample. Prior to the electrochemical test, the samples were again annealed at 850° C. for 4 h with excess 5 mol % of Li2 CO3 after hydrothermal relithiation. As shown in
It should be noted that besides the replacement of 4 M LiOH with a mixture of 0.1 M LiOH and 3.9 M KOH, the cost of hydrothermal relithiation can be also reduced by recycling the concentrated LiOH solution. To briefly evaluate the process effectiveness, a D-NMC111 sample was relithiated with a previously used 4 M LiOH solution at 220° C. for 2 h, annealed at 850° C., and subjected to electrochemical cycling for comparison. The initial discharge capacity sample was found to be 156.6 mAh/g and 150.5 mAh/g at C/10 and C/3, respectively, also retaining 92.8% of this capacity after 50 cycles, comparable to T-NCM111. The viability of continuous recycle/reuse of the spent LiOH during relithiation also represents a promising strategy in reducing the recycling cost based on the hydrothermal relithiation method.
As described above, after hydrothermal relithiation, a short annealing is required to remedy the remaining structural defects and achieve the desired electrochemical performance. However, during high-temperature annealing, a portion of Li in the crystal structure may be lost, leading to inferior capacity and cycling instability (
When LiOH was used as the Li source instead, the relative intensity ratio of Ni 2p to O 1s for the samples annealed at 550° C. and 650° C. was not as low as that of the sample annealed with Li2 CO3 under the same conditions (
The electrochemical performance of the samples annealed at different conditions were then compared with the same protocol described earlier. When the relithiated sample was annealed at 550° C., the capacity of the first charging cycle at C/10 was 231.4 mAh/g, far beyond that of pristine T-NCM111 (182.8 mAh/g). The discharge capacity was only 154.6 mAh/g, corresponding to a Columbic efficiency (CE) of 66.8%, which again is likely from the irreversible Li+ capacity introduced by residual Li2 CO3. However, after washing with water, the charge capacity dropped to 185.4 mAh/g, indicating that the additional capacity originated from the residual Li2 CO3. When the annealing temperature was increased to 650° C., the charge and discharge capacities dropped to 212.2 mAh/g and 152.3 mAh/g, respectively, corresponding to a CE of 71.7%, indicating that the inactive Li2 CO3 remained in the cathode material. Even when the temperature was further increased to 750° C., the charge capacity was still as high as 206.4 mAh/g but the discharge capacity was only 149.8 mAh/g. When the annealing temperature was increased to 850° C., the charge capacity dropped to 182.2 mAh/g, and the discharge capacity increased to 156.8 mAh/g, achieving a CE of 86.1%, which is even higher than that of pristine T-NCM111 (85.6%).
The cycling stability was also compared for these samples annealed with Li2 CO3 at various temperatures (
The samples annealed at 550° C. and 650° C. showed a similar capacity degradation trend where the capacity retention was improved from 88.2% to 90.4%. Increasing the annealing temperature to 850° C. further increased the initial capacity to 150.4 mAh/g and a capacity of 140.7 mAh/g could be achieved after 50 cycles, (a capacity retention of 93.1%), which is comparable to pristine T-NCM111. Therefore, it was concluded that 850° C. is the minimum temperature required for fully restoring the electrochemical performance of D-NCM111 when Li2 CO3 is used as the Li source.
The optimal annealing time at 850° C. was evaluated by testing the cycling stability of relithiated samples annealed at 850° C. for 1 h, 2 h, 4 h and 6 h. After annealing for 1 h, the sample exhibited a capacity of 149.1 mAh/g, which decreased to 130.2 mAh/g over 50 cycles. With a further increase of annealing time to 2 h, the initial capacity was not affected dramatically (150.2 mAh/g). Nevertheless, the capacity after 50 cycles was improved to 135.6 mAh/g, which increased further to 140.3 mAh/g when the sample was annealed for 4 h. Annealing for 6 h did not improve the capacity or cycling stability, leading to the conclusion that a duration of 4 h is sufficient to attain the desirable electrochemical performance.
Due to the lower melting point (471° C.), LiOH was proposed as an alternative to Li2 CO3. Similarly, different annealing temperatures were explored to identify the optimal temperature for fully recovering the electrochemical properties of the regenerated cathode. When the annealing temperature was 550° C., the charge capacity reached 180.3 mAh/g, which was significantly lower than that of the sample annealed under the same condition with Li2 CO3 as the Li source (231.4 mAh/g). The reversible capacity was 154.4 mAh/g, corresponding to a CE of 85.6%. This indicates that Li+ from LiOH can be more easily incorporated into the layered crystal structure at a low temperature compared to Li2 CO3. The charge and discharge capacities did not change when the temperature was increased to 650° C. The increase of temperature to 750° C. improved the reversible capacity to 156.7 mAh/g, almost identical to that of the sample annealed at 850° C. (156.8 mAh/g).
The cycling stability of all the samples annealed with LiOH is shown in
The T-NCM and R-NCM electrodes after 50 cycles were characterized by XPS and XRD to probe the difference in structural durability. The overall XPS survey spectra of cycled T-NCM and R-NCM (
Additionally, the direct regeneration was scaled up from 1 g to 10 g of cathode per batch to demonstrate the viability of the process. Referring to Table 11, the composition of the product (R-NCM-10 g) was analyzed by ICP, where the Li content was found to be similar to that of the control sample T-NCM111, showing no difference from the sample regenerated at a smaller scale (R-NCM-lg). The composition of NCM111 regenerated at different scales and the control samples (D-NCM111 and T-NCM111).
The R-NCM-10 g was further characterized by XRD and XPS. Compared with the starting D-NCM, a right shift of the (003) diffraction peak (
The inventive scheme provides for the regeneration of chemical composition and microstructure in degraded Li-ion battery cathodes using a novel particle-to-particle approach that is economical, energy efficient and more environmentally friendly than existing methods. Hydrothermal treatment with a short annealing step provides the regenerated cathode materials with high capacity, long cycling stability and high rate performance comparable to that of pristine materials.
This is a continuation-in-part of application Ser. No. 16/960,284, filed Jul. 6, 2020, which is a 371 national phase filing of International Application No. PCT/US2019/012572, filed Jan. 7, 2019, which claims the benefit of the priority of U.S. Provisional Applications No. 62/614,300, filed Jan. 5, 2018, and No. 62/682,822, filed Jun. 8, 2018, each of which is incorporated herein by reference in its entirety.
This invention was made with government support under Grant No. DE-AC02-06 CH11357 awarded by the United States Department of Energy. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 62682822 | Jun 2018 | US | |
| 62614300 | Jan 2018 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 16960284 | Jul 2020 | US |
| Child | 17697889 | US |
