Facile Preparation of High Performance Low Concentration HCHO Degradation Catalyst from Waste Li-MnO2 Batteries

The recycling and utilization of lithium-ion batteries has received a lot of attention. The use of recycled waste lithium-manganese batteries to degrade formaldehyde contaminated gas by adsorption is certainly killing two birds with one stone. In this paper, efficient catalysts capable of degrading formaldehyde were obtained using lithium-manganese button batteries being discharged to different levels and then recovering the cathode material by a simple method and labelled as LixMnO2 (x = 0.00; 0.25; 0.50; 0.75; 1.00). The fully discharged cathode material Li1.00-MnO2 degraded formaldehyde at nearly 100% (less than 0.1 ppm) within 24 h at room temperature, which is twice the degradation rate of the undischarged cathode material. The high degradation efficiency is attributed to the continuous doping of Li+ as the discharge proceeds and the conversion of Mn(IV) to Mn(III), so the lattice gap, defects, surface oxygen species and specific surface area of the catalyst increase. And the surface oxygen involved in the degradation of formaldehyde increases. The catalytic activity of the catalyst for formaldehyde gradually increased with the discharge, promoting the catalytic degradation effect. The degradation rate of formaldehyde at low concentrations was close to 100% within 24 h. This study provides an attractive approach for converting lithium battery electrode materials into formaldehyde degradation catalysts to improve the indoor environment.


Introduction
Formaldehyde is one of the typical indoor environmental pollutants, ranking second in China's toxic chemicals priority control list [1]. In June 2004, the International Agency for Research on Cancer raised formaldehyde from a secondclass carcinogen to first-class of carcinogen [2]. With the accelerated development of urbanization, formaldehyde is abundantly produced in the adhesive of interior decoration materials [3]. Formaldehyde in artificial flooring can release as long as 15 years [4,5]. According to the survey, most people spend 80% of their daily lives indoors, while older people and children spend more time indoors [6]. Exposure to formaldehyde can cause infections in the upper respiratory tract and lungs. Prolonged exposure can lead to abnormal liver function, immune function, nervous system and even cancer [7][8][9][10]. People will feel irritation and discomfort of the nasal cavity when the indoor formaldehyde concentration reaches 0.1 mg/m 3 . As the concentration increases, the onset of symptoms is cough, chest tightness, nausea, vomiting, and it could cause death when reaching 30 mg/m 3 [11,12]. At present, the method for reducing the concentration of formaldehyde is mainly activated carbon adsorption. As a physical adsorption, it can only transfer the source of pollution, not fundamentally decompose the source of pollution [13][14][15]. In recent years, the use of photocatalytic degradation of formaldehyde has become a research hotspot [16][17][18]. Formaldehyde is oxidized to form CO 2 and H 2 O by holes (h + ) and ·OH in the photocatalytic oxidation method [19][20][21]. However, noble metal photocatalysts such as Ptloaded TiO 2 need to be irradiated by a light source (mainly ultraviolet light) and the cost of catalysts is expensive [22,23].
Metal oxide method refers to the decomposition of formaldehyde and other organic substances into harmless substances by the strong oxidation and surface properties of metal oxides without the need of light. Manganese oxide (MnO x ) has been investigated to degrade many gaseous pollutants in the environment, especially for indoor air purification. MnO 2 exhibits catalytic oxidation characteristics due to the uniqued orbital transition of the outer electrons between different energy levels. Yoshika and Sekin [6] loaded activated carbon granules and manganese oxide (77% commercial MnO 2 ) onto the fibers with a binder that converts formaldehyde to CO 2 at room temperature. Sekin [24] also compared the removal efficiency of formaldehyde by Ag 2 O, MnO 2 , TiO 2 , CeO 2 , CoO, Mn 3 O 4 and other catalysts. Sidheswaran [25] investigated the effect of MnO x materials on their activity for complete oxidation of formaldehyde. Miyawaki [26] loaded MnO x onto activated carbon fibers to mineralize formaldehyde to CO 2 . MnO 2 has been widely studied as a catalyst for formaldehyde degradation, however its catalytic efficiency is low and easy to be deactivated at room temperature [27,28]. Zhang [29] reported lignocellulose-based activated carbon fiber paper (LACFP) loaded with manganese dioxide (MnO 2 ) was fabricated for the adsorption and in-situ catalytic degradation of formaldehyde. The in-situ catalytic degradation of formaldehyde results indicated that MnO 2 -loaded LACFP could catalyze formaldehyde into CO 2 and H 2 O. Sherif [30] have reported on a new approach of sustainable fabrication of mono-dispersed MnO 2 particles (20 nm average particle size) with a constant product quality using hydrothermal synthesis procedures. Because of the semiconductor characteristics of MnO 2 itself and the large specific surface area of the synthesized sample, which is very suitable for the degradation of organic pollutants as a photocatalyst.
MnO 2 , as the cathode material, is widely used in lithium manganese battery. Due to the irreversible phase change and structural strain in the discharge process of lithium manganese battery, the battery life is lost. The discharge formula of lithium manganese battery is shown as follows: With the discharge of the battery, Li + is gradually embedded into the lattice of MnO 2 , changing the lattice structure of MnO 2 , the specific surface area, the state of oxygen and the charge density of MnO 2 [31][32][33][34]. It is also accompanied by the transition of Mn (IV) to Mn (III), the Mn (III) ion is a Jahn-Teller distorted ion that tends to break symmetric orbitals, resulting in lattice dilation and structure degradation [35][36][37][38]. The intermediate state of Mn (III) as a catalytic reaction can lead to an increase in catalytic effect [39], which will make waste batteries have higher formaldehyde catalytic efficiency compared with ordinary MnO 2 . (1) Every year, a large number of waste lithium manganese batteries are produced, which also leads to environmental pollution. At present, the recovery methods of lithium manganese batteries can be divided into pyro-metallurgy, hydrometallurgy, and bio-metallurgy [40][41][42]. These methods have high recovery cost [43], complex steps [44], and will cause secondary pollution [45]. Therefore, the development of a simple cathode material recycling method is important.
Herein, we demonstrate the cathode material of lithium manganese battery after depletion through simple steps of disassembling, washing, filtering and drying, and obtained a catalyst that can be directly used to degrade low concentration formaldehyde at room temperature. The catalyst was labeled as Li x MnO 2 (x = 0.00, 0.25, 0.50, 0.75, 1.00) according to the degree of discharge. With the progress of discharge, the catalytic activity of lithium manganese dioxide on formaldehyde increased gradually and the catalyst obtained by complete discharge maintained 100% catalytic efficiency on formaldehyde within 24 h. Then the detail on the changes of cathode materials in the discharge process were studied by XRD, HR-TEM, BET, XPS, and TOC for its phase structure, oxidation state and surface oxygen structure. Correspondingly, the mechanism of degradation of formaldehyde by catalysts was discussed. This paper provides a simple method to prepare formaldehyde degradation catalyst by recycling cathode material of lithium manganese battery.

Simulated Discharge of Commercial Li-MnO 2 Button Batteries
The lithium manganese battery which used in this experiment is a Panasonic CR2032 coin cell battery with a standard voltage of 3 V, a standard capacity of about 210 mAh, and a weight of about 3 g.

Disassembly of Lithium Manganese Button Battery and Recovery of Cathode Material
The lithium manganese button battery was simply disassembled and recovered to obtain a catalyst for directly decomposing formaldehyde at room temperature. Firstly, the small-scale hydraulic button battery-dismantling machine was used to disassemble the battery in the glove box under an argon atmosphere, the positive and negative electrodes were separated and the cathode material was retained. The remaining stainless steel casing and nylon sealing ring can be directly collected, which the secondary pollution can be avoided. The obtained solid was weighed, manually crushed and grinded, and washed with 300 ml/g of deionized water to pH = 7. In order to fully edulcoration the electrolyte, each cleaning time was 30 min. Secondary, the solid powder was recovered by filtration and the washed waste liquid was treated, then the obtained powder was vacuum dried at 60 °C for 12 h. Finished, the dried solid can directly be used as a catalyst for formaldehyde degradation at room temperature.

Catalytic Activity of Formaldehyde Decomposition
The HCHO removal test was performed in a quartz tube reactor at ambient temperature. The 0.1 mol/L formaldehyde solution vessel was maintained at 0 °C, and a mixture of 40 sccm of O 2 and 160 sccm of N 2 was passed through it to produce gaseous HCHO. The gas was passed through a quartz tube reactor filled with 500 mg of Li x MnO 2 (x = 0.00, 0.25, 0.50, 0.75, 1.00), which wrapped with quartz wool. The weight hourly space velocity (WHSV) is fixed at 24,000 mL/g/h. The reaction temperature was maintained at 25 °C. The initial concentration of HCHO was fixed at about 4 ppm and monitored by the HCHO detector. Quartz tube reactor inlet and outlet gases were monitored by a PPM-400 ST HCHO detector (detection range and limits were 0-20 ppm and 20 ppm, respectively). The degradation of HCHO was calculated according to the following Eq. (2). Where HCHO 1 is the initial HCHO concentration and HCHO 2 is the HCHO concentration at different time after passing through catalyst.

Characterization of Recycled Cathode Material
The crystal structure of samples was collected by Powder X-ray diffraction (XRD, Smart Lab SE, Japan) with Cu Kα radiation at 40 kV and 40 mA. A scanning step of 0.02° and scanning speed of 0.2°/s were applied in the 2θ range of 10-80°. High-resolution transmission electron microscopy (HR-TEM) images of the catalysts were taken on the FEI TECNAI G2 F20 instrument. The physical adsorption isotherm for the sample was tested by the ASAP 2020 specific surface and porosity analyzer produced by Micromeritics Instruments, Inc., and the adsorption gas is nitrogen. The specific surface area of the material was tested by BET (Brunauer-Emmett-Teller) method, and the pore size distribution and total pore volume of the material were characterized by BJH (Barrett-Joiner-Halenda) model. The total pore volume was calculated at a relative pressure of 0.99. X-ray photoelectron spectroscopy (XPS) analysis was conducted on an ESCALAB 250 photoelectron spectrometer (Thermo VG Scientific, USA). Monochromated Al target, dual anode Al/Mg target was selected as the X-ray source target, and the energy resolution was 0.45 eV/(Ag 3d5/2), 0.82 eV/(C 1 s). The total organic carbon (TOC) was calculated by Shimadzu total organic carbon analyzer TOC-V and the solid sampling device SSM-5000A. The total organic carbon content of the samples before and after the reaction was determined by subtraction method (TOC = TC − IC).

Characterization and Analysis of Recycled Cathode Material from Li-MnO 2 Batteries
The XRD patterns of Li x MnO 2 obtained from recovering Li-MnO 2 button battery cathode material are shown in Fig. 1 [44,45]. The product shows no characteristic peaks for the other manganese valence states, indicating that no impurities of other valence states are present.
With the discharge of the battery, the structure of the cathode material changes step by step during the transformation from Li 0.00 MnO 2 to Li 1.00 MnO 2 . As shown in Fig. 1, due to the electrochemical intercalation of Li + , the main changes of XRD pattern are as follows: (i) The characteristic peak (a) of 2θ = 18.5° appears and gradually strengthens, while the new characteristic peak (e) appears and moves stage by stage towards a lower angle with discharge; (ii) The gradual disappearance of characteristic peaks (b) and (f); (iii) The peaks of (c), (d) and (g) gradually shifts to a lower angle as the discharge progresses, and the intensity remains. At the same time, the carbon peak remains unchanged during the continuous discharge. These changes indicate that the structure of the cathode material has irreversible changes due to continuous discharge and lithiation. The appearance of peaks (a) and (e) has been reported [42,46,48], while the crystal structure of the cathode material after full discharge cannot be determined. Peak (a) attributes to some representative compounds containing Li + , such as LiMn 2 O 4 , LiMnO 2 , Li 2 MnO 3 [49][50][51], which is caused by the gradual embedding of Li + into the lattice of MnO 2 during discharge. The disappearance of peaks (b) and (f) is consistent with reports in the literature [52,53]. Li 2 MnO 3 belongs to monoclinic space group C2/m, while LiMnO 2 crystallizes in trigonal system group R-3 m. Both of them are constructed by alternately stacking Li layers and transition metal (TM) layers along c axis within the similar cubic close packed oxygen array.
Due to the insertion of Li + , the lattice structure of MnO 2 gradually changes, during the discharge, also some characteristic peaks disappear along with the partial electrochemical reduction from Mn (IV) to Mn (III). The shift of characteristic peaks may be correlated with the lattice expansion of MnO 2 , the larger radius of Mn (III) and Li + than Mn (IV) or the oxidation transition from Mn (IV) to Mn (III) which results in transformation of phase structure was known as Jahn-Teller distortion [46,54]. These results indicate that the lithiation process changes the crystal structure of MnO 2 , and with the embedding and dispersion of Li + , the properties of MnO 2 are gradually changed.
HR-TEM was used to observe the lattice spacing and microcrystalline plane on the catalyst surface. As shown in Fig. 2, the lattice spacing of MnO 2 gradually increases with the enhancement of lithiation, which is consistent with the low-angle migration of individual peaks in XRD. At the same time, the discharge also increases the complexity of the cathode material, gradually presents lattice distortion and dislocation. Due to the insertion of Li + and the transformation from Mn (IV) to Mn (III), the lattice of the original MnO 2 gradually expands, which can expand more active sites for the catalytic reaction. In order to further explore the impact of Li + embedding swelling on cathode materials, BET analysis was used as an important method to calculate the specific surface area, pore size and pore volume of cathode materials. As shown in Fig. 3, in the low-pressure region, reversible monolayer adsorption mainly occurs, and there is a certain hysteresis phenomenon. Therefore, the isotherm type of adsorption and desorption can be judged as type IV, and the H3 type hysteresis loop indicates that the sample has small mesoporous. In addition, capillary condensation only occurs when the pressure of the sample is close to the saturated vapor pressure, and the adsorption and desorption curves show a slow decline rather than a rapid decline like the parallel plate aperture curves. Therefore, it is presumed that the pores are mainly slit holes generated by inter-partical accumulation. As shown in Table 1, the surface area, pore volume and pore diameter of Li 0.00 MnO 2 , Li 0.25 MnO 2 , Li 0.50 MnO 2 , Li 0.75 MnO 2 and Li 1.00 MnO 2 catalysts were obtained according to the calculated N 2 adsorption and desorption isotherm data. With the occurrence of discharge, the specific surface area of the sample increases gradually. Compared with the initial sample (Li 0.00 MnO 2 ), the surface area of the fully discharged Li 1.00 MnO 2 increased by 65.3%, reaching the maximum value of 45.86 m 2 /g. The above results indicate that continuous discharge causes Li + to be gradually embedded, resulting in lattice distortion and lattice expansion, thus increasing the specific surface area of the sample. As the discharge progresses, the structure and performance of the cathode material were adjusted, and the larger specific surface area and higher porosity are conducive to promote the mass transfer of formaldehyde and improve the adsorption/ storage capacity of the catalyst for formaldehyde, in turn it also improve its catalytic degradation activity of HCHO.
As an electrochemical reaction occurs during the discharge process, the TOC test results are shown in Fig. 4, in order to explore the influence of residual organic matter and carbon additives on the catalysis before and after the reaction. It can be seen evidently from Fig. 4, the content of TOC increases as the discharge proceeds, which is due to the further penetration of electrolyte and organic solvent during the process of Li + embedding into the cathode material. However, the content of the cathode material after formaldehyde degradation reaction was lower than that before the reaction, which may be because some C-containing groups on the surface of the cathode material participated in the catalytic reaction and generateed CO 2 production. It was found from the TOC content that that carbon additive in the cathode material did not occur mineralization, ruling out the influence of carbon additive on the formaldehyde degradation reaction.
To investigate the changes in the elements on the surface of the samples after full and insufficient discharge, the elemental composition, metal oxidation state and adsorption species of cathode materials were investigated by XPS spectra. With different lithium levels, the Li 1 s, Mn 2p, Mn 3 s and O 1 s spectra of cathode materials are shown in Fig. 5. As the discharge proceeds, a small amount of Li + has the potentional to enter the lattice gap and become a kind of    (Fig. 5a), which is the peak of Li + in the oxide [55]. The embedding of Li + contributes to adsorption of water between interlayers and promotes the oxidation of formaldehyde. Meanwhile, Mn 2p orbital (Fig. 5b) has two peaks, Mn 2P 3/2 and Mn 2P 1/2 , whose spin energy is separated into 11.6-11.7 eV, which is consistent with the reported MnO 2 spectrum [56,57]. The 2p 3/2 peak of Mn 2p orbital moved towards the lower binding energy as the discharge proceeds, which indicates the decrease of Mn average valence state. As shown in Fig. 5c, the increase of Mn 3 s orbital peak difference also reflects the decrease in Mn valence state. Mn 3 s peak splitting results from parallel spin coupling between electrons in the 3 s and 3d orbitals, and the splitting width increased as Mn valence decreased due to the presence of additional electrons in the 3d orbital [58]. According to the Fig. 5c, the average oxidation (AOS) of Mncan can be further calculated from the difference multi-peak splitting (ΔE Mn 3 s ), Which is shown on the following formula [59]: The details of Mn 3 s binding energy (BES), ΔE Mn 3 s and AOS are summarized according to Fig. 5c (Table 2). On discharge, the average valence state of Mn decreased from 3.71 to 3.05, which is consistent with the total discharge equation of Li-MnO 2 battery Eq. (1). The low average oxidation state of completely discharged Li 1.00 MnO 2 is due to the increase of Mn (III) content from the lithium process. The increase of Mn (III) content also indicates the increase of Mn (III) /Mn (IV) content in MnO 2 , which result in an increase in surface defects while also promoting the catalytic activity of cathode materials.
In order to further study the effect of discharge degree on catalytic performance of cathode materials, the distribution and change of oxygen species in O 1 s were investigated. As shown in Fig. 5d, the O latt peak around 529 eV is mainly the type of lattice oxygen, while the O surf peak around 531 eV is the surface oxygen and oxygen defect [60,61]. O latt and O surf species are considered to be reactive oxygen species for free radical generation and oxidative decomposition, among which, surface oxygen and oxygen defects play a key role in catalytic oxidation of formaldehyde due to their higher mobility [62]. The ratio of lattice oxygen decreased from 59.24% to 26.36%, and the surface oxygen and oxygen defects increased from 40.76% to 73.63% with the discharge. The high Olatt/Oads molar ratio could facilitate the catalytic degradation of VOCs such as benzene, ethylbenzene, toluene, and o-xylene [63,64]. Formaldehyde degradation requires hydroxyl radicals, which derived not only from oxygen or water electrolyte reactions, but also related to solid electrolyte interface layer formed on the surface of manganese dioxide. This indicates that a solid redox pair of Mn (III)/Mn (IV) is likely to form on the surface of MnO 2 [65], and the decrease in the relative amount of O latt may be accompanied by the formation of a noncondensed oxide structure [66], which also explains the reduction and disappearance of individual peaks in XRD.

Catalytic Performance of Recycled Cathode
Material from Li-MnO 2 Batteries.
The removal efficiency of catalysts before and after complete discharge at room temperature was shown in Fig. 6. The degradation of HCHO was calculated according to Eq. (4): where HCHO 1 is the initial HCHO concentration and HCHO 2 is the HCHO concentration at different time after passing through catalyst. The degradation efficiency of formaldehyde in the cathode electrode material before discharge (Li 0.00 MnO 2 ) was reduced to 50% within 10 h, with the electrochemical insertion of Li + and discharge, the degradation efficiency of formaldehyde is gradually improved. The cathode electrode material after complete discharge maintained nearly 100% degradation efficiency at room temperature (25 °C) during the whole test period (24 h), which the concentration after degradation is less than 0.1 ppm. The inlet HCHO concentration of ~ 4 ppm is about forty times of the allowable exposure limits for HCHO in China, representing the extreme high HCHO pollution. As mentioned above, the XPS results (Fig. 5) indicate that the amount of adsorbed surface oxygen species increased with Li + is intercalated into MnO 2 . These results clearly showed the catalytic activity for HCHO decomposition is governed by amount of adsorbed surface oxygen species, and the addition of Li + greatly enhanced their amount and activity. At the same time, the embedding of Li + improved the specific surface area of the cathode material, enhanced its adsorption to formaldehyde, oxygen and water molecules, provided more reaction sites. The increase of reactant concentration on catalyst surface contributes to the degradation of formaldehyde, which is also consistent with the increase of O surf proportion in XPS results (Fig. 5). These results indicate that the catalytic performance of Li x MnO 2 for formaldehyde enhances with the discharge process of lithium manganese cathode material. The completely discharge Li 1.00 MnO 2 in actual indoor environment on an experiment of HCHO removal (4) HCHO degradation(%) = HCHO1 − HCHO2 HCHO1 × 100% has good activity and stability, which can be attributed to Li + doping under caused by adsorption material surface oxygen activity and more reaction sites.

Exploration of the Catalytic Mechanism
The catalytic stability of Li 1.00 MnO 2 before and after formaldehyde degradation was tested by XRD analysis (Fig. 1b). The catalyst after 24 h reaction is consistent with that before the reaction, which shows the good stability of the catalyst. The beta-MnO2 belongs to n-type semiconductor and includes some oxygen deficiency, which its composition is not completely consistent with stoichiometry [26]. When the adsorption oxidation oxygen or other gas, there will be a smaller degree of adsorption to supplement the oxygen vacancy [6]. With the progress of discharge and electrochemical reduction of Mn (IV), Li. + gradually occupied the empty tunnel of MnO 2 and changed the structure of MnO 2 , such as oxidation state, phase structure, formation of adsorbed species on the surface, and further improved the catalytic activity of the material. Lithiation reaction formula is as follows [67] In the formula, (·1)indicates the position of the hollow structure of the tunnel, and [] indicates the position of the octahedron occupied by (1 × 1) Mn. Due to the electrochemical embedding of Li + , this will disrupt the electronic state of the active center, thus increasing the number of electronabsorbing groups, the valence state of Mn decreased, and the Mn (III) content increased, which increased the surface deformation degree of the cathode material. The generation of more reaction potential can increase the catalytic activity of the catalyst. Formaldehyde degradation reaction formula shows the general reaction pathway for the catalytic decomposition of formaldehyde by MnO 2 at room temperature. Formaldehyde can be adsorbed on MnO 2 surfaces by forming hydrogen bonds with surface adsorbed water or surface hydroxyl groups. The adsorbed formaldehyde or its hydrate (H 2 C(OH) 2 ) is then oxidised by surface reactive oxygen species (-O2 − , -O − , -OH) successively to DOM, formate, carbonate and eventually CO 2 to be desorbed from the MnO 2 surface. The oxidation of formaldehyde consumes reactive oxygen species on the surface, creating oxygen vacancies. Oxygen molecules in the air will adsorb to the oxygen vacancies and dissociate to produce new surface active oxygen. The surface hydroxyl groups consumed by the oxidation of formaldehyde are also replenished by the dissociation of H 2 O from the oxygen vacancies. The reaction mechanism shows that for the low temperature catalytic oxidation of formaldehyde, surface adsorbed oxygen species (-O 2 − , -O − ), adsorbed water and hydroxyl groups play a key role in the reaction [68]. The introduction of Li + enhances the activity of surface oxygen, which can react with H 2 O, thus compensating for the consumption of -OH and making the oxidation of formaldehyde sustainable. While the catalytic decomposition of formaldehyde is significantly influenced by the content of Mn vacancies in the metal. Manganese vacancies lead to the presence of a large amount of surface adsorbed oxygen. At the same time, the presence of Li + near the Mn vacancies promoted the activation of these surface adsorbed oxygen groups and enhanced the catalytic activity of M formaldehyde. The surface defects of the catalyst are not only point defects but also surface defects. Formaldehyde degradation reaction formula is as follows [6,26,69]: The increase of surface hydroxyl and Mn (III) boosted the catalytic activity of recovering manganese dioxide, while the increase of specific surface area provided more reaction sites, showing excellent formaldehyde degradation performance of cathode materials after discharge.

Conclusions
In this study, the cathode materials of lithium manganese batteries with different discharge degrees were recovered by simple method, and the formaldehyde with low concentration was catalyzed and degraded at room temperature. Through the discharge, the structure composition, oxygen species and Mn valence state of the catalyst were changed. Meanwhile, as the discharge progressed, Mn (IV) was gradually converted into Mn (III), the lattice gap of the catalyst the defects and surface oxygen species, and the specific surface area gradually increased. Rich reactive oxygen species, abundant Mn (III) species and altered structure were very conducive to improving catalytic activity. The catalytic activity of the catalyst on formaldehyde enhanced with the discharge, and the cathode material Li 1.00 -MnO 2 with complete discharge had a higher specific surface area and water absorption rate of the active part. The catalyst after complete discharge had a nearly 100% degradation efficiency of formaldehyde at room temperature in 24 h. This study provides an attractive method for converting depleted lithium battery electrode material into formaldehyde degradation catalyst and a potential solution for waste lithium manganese battery recycling and indoor air treatment.