Battery waste-derived functional materials for the capture and removal of harmful gases

Abstract

The persistent use of primary alkaline batteries in electronic gadgets and lithium-ion batteries in electric vehicles is creating a large volume of battery waste. Proper management and processing are necessary to prevent the dumping of used batteries in landfills. Valuable metals such as lithium, cobalt, nickel, and zinc can be extracted and purified from spent batteries. Alternatively, they can be used in synthesising functional materials. This review explores a promising solution for battery waste management by repurposing it to create materials capable of removing harmful gases. Reusing battery components such as electrodes, electrolytes, and polymer separators leads to the development of innovative strategies for creating adsorbents and catalysts. These materials are capable of efficiently capturing or catalysing harmful gases into harmless gases or ions. The review outlines various methods for converting battery waste into valuable materials, structural modifications, performance evaluations, and underlying mechanisms responsible for the removal of harmful gases. This review highlights the potential of battery waste as a sustainable resource for addressing rising air pollution and promoting a circular economy.

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Nishesh Kumar Gupta

Dr Nishesh Kumar Gupta is currently a SERB-funded National Postdoctoral Fellow at the Indian Institute of Science Education and Research Bhopal, India. He earned his PhD from the University of Science and Technology, Korea and Integrated MSc from the National Institute of Technology Rourkela, India. His current research focuses on the synthesis of metalated COFs for CO2 fixation. He has authored more than 70 peer-reviewed articles, secured 5 patents, and contributed to 4 book chapters, accumulating over 1900 citations. His achievements have led to recognition on the Stanford List of Top 2% World Scientists for both 2022 and 2023.

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Environmental significance

Repurposing battery waste for toxic gas removal minimizes environmental harm from electronic waste and mitigates air pollution. Transforming discarded battery components into functional materials reduces the reliance on raw materials and enhances air quality by efficiently neutralizing toxic gases. This innovative approach aligns well with the circular economy principles, promoting sustainable battery waste management while addressing severe environmental challenges.

Introduction

Given the increasing global demand for clean energy, managing waste from energy storage systems, particularly batteries, has become a major environmental concern. Primary alkaline batteries like Zn–MnO₂ batteries are extensively used to power portable electronic devices. However, post-exhaustion, billions of these single-use batteries end up in landfills, posing severe threats to the environment. Moreover, dumping these batteries in landfills leads to a significant loss of metal resources such as Zn and Mn.¹ Rechargeable batteries like lithium-ion batteries (LIBs) are considered the backbone of the electric vehicle (EV) industry. The need to transition from fossil fuel-powered vehicles to EVs is driving LIBs production at an unprecedented scale. It is projected that annual production will surpass the ‘one million tons’ mark by 2025.² The International Energy Agency estimated that EVs manufactured in 2019 alone generated 500 000 tons of LIBs waste. By 2040, this figure could escalate to a staggering 8 million tons of waste.³ With improper disposal mechanisms, most spent LIBs are either temporarily stored or end up in landfills, causing severe contamination of the land and local ecosystem. These spent LIBs are valuable commodities, as the elements required for making the cathode of LIBs, including Li, Co, Ni, and Cu, hold significant market value.⁴

Recycling is considered the most sought-after method for extracting valuable metals from spent batteries.^5–8 However, battery recycling faces severe restrictions due to limited metal recovery, non-uniformity in cathode composition, overuse of acids, bases, and organic solvents, high energy costs, and the emission of toxic gases during processing.^9,10 Another approach to reducing battery waste is repurposing it into valuable functional materials, offering a promising avenue for upcycling. This could provide significant commercial value to battery waste before recycling, thereby reducing overall recycling costs. In the literature, numerous reports have demonstrated promising applications of battery waste-derived functional materials in heterogeneous catalysis,^11,12 energy storage devices,^13,14 and heavy metal sequestration.^15,16 Liu briefly reviewed these applications in 2018.¹⁷ However, the review may be outdated, as battery-derived functional materials have since found new applications in gas capture and removal. Thus, there is a need for a review to highlight the diversified use of battery waste-derived functional materials for air decontamination applications.

This review focuses on the emerging field of battery waste-derived functional materials, specifically highlighting their applications in capturing and removing harmful gases. Emissions from industrial facilities, thermal power plants, incineration sites, and vehicles contain a mixture of harmful gases, including sulphur oxides (SO_x), nitrogen oxides (NO_x), carbon oxides (CO_x), volatile organic compounds (VOCs), and hydrogen sulphide (H₂S). The complex interaction of these gases in the atmosphere, influenced by solar radiation and water vapour, leads to the formation of photochemical smog and acid rain, contributing to climate change and posing threats to both human health and the environment. Prolonged exposure to these airborne pollutants is associated with various adverse health outcomes, such as asthma, lung cancer, psychological disorders, autism, and low birth weight.^18,19 Addressing this urgent issue requires the development of effective and cost-efficient technologies for capturing and eliminating gaseous contaminants. By utilizing the inherent properties of battery waste constituents, including cathodes, polymers, anodes, and electrolytes, researchers are exploring novel opportunities to develop sustainable materials suitable for treating harmful neutral and acidic gases. This review analyses recent progress in the synthesis, characterization, and performance assessment of battery waste-derived functional materials for treating VOCs, toxic neutral gases (CO and NO), and acidic gases (CO₂, NO₂, SO₂, and H₂S). This review aims to highlight the concept of ‘waste-to-wealth’ in the context of battery waste, which could set a precedent for processing future battery wastes, including those from newly developed LIBs and sodium-ion batteries (NIBs).

Battery waste-derived functional materials

Batteries are typically divided into two categories, i.e., primary and secondary batteries. The key difference lies in the nature of their chemical reactions. Primary batteries, such as Zn–MnO₂ and Zn–carbon batteries, are discarded once they can no longer provide sufficient electrical energy. In contrast, secondary batteries, like LIBs and NIBs, can convert chemical energy into electrical energy through reversible chemical reactions. This means the original chemical state can be restored by reversing the current flow, i.e., by charging from an external source. However, with continuous use, even secondary batteries experience performance fading and are discarded when their performance reaches a minimum threshold. These LIBs are poorly collected and recycled, resulting in most of them being disposed of in landfills.

Alkaline Zn–MnO₂ batteries and LIBs are frequently used for synthesising functional materials. While multi-step synthetic approaches are commonly employed in developing these materials, some studies have reported the direct use of battery waste without significant pre-treatment. Here, a brief discussion has been included to inform readers about conventional approaches that have been adopted for developing functional materials derived from battery waste with some details about synthesis processes.

While discharging a Zn–MnO₂ alkaline battery, Zn oxidizes to produce ZnO, while MnO₂ undergoes reduction to form MnOOH and Mn(OH)₂. The MnOOH reacts with Mn(OH)₂ and Zn(OH)₄²⁻ to yield Mn₃O₄ and ZnMn₂O₄, respectively.²⁰ Thus, the black mass derived from spent alkaline Zn–MnO₂ batteries is rich in ZnO and MnO₂ phases with minor presence of other metal and metal oxides, such as Zn, MnO, Mn₂O₃, Mn₃O₄, and ZnMn₂O₄, along with KOH electrolyte and graphitic carbon.²¹ This black mass could be modified based on the requirements of the target gas molecule and the treatment process. For capturing highly acidic gases like H₂S, SO₂, and NO₂, the KOH electrolyte along with the ZnO–MnO₂ composite in the black mass is sufficient for gas oxidation, and thus, no pre-treatment is required.^21,22 However, for the thermal oxidation of gases like VOCs, researchers have treated the black mass with strong mineral/organic acids either to neutralize the KOH electrolyte^23–25 or to completely/selectively dissolve metals.^26,27 In some cases, a hydrometallurgical process like bioleaching is used to eliminate the use of strong mineral acids.^28,29 The leached metal solution was further processed to obtain the desired functional materials like Zn_xMn_3−xO₄,³⁰ MnO₂,²⁷ and Mn₂O₃.²⁶ Further modifications have been made to the black mass-derived materials to improve the catalytic properties, such as by adding photoactive TiO₂,³¹ and catalytically active metals like Pd,^32–34 Cu,²⁷ and Ag.³⁵ Doping functional materials with active metals can improve physicochemical properties, such as surface area,²⁷ and lower the activation energy barrier associated with the catalytic elimination of gaseous pollutants.

The metal leachate obtained from the treated black mass of Zn–MnO₂ batteries could be used as a metal solution for developing new oxide materials, providing better control over the phase composition, morphology, and other desirable properties like surface area and porosity. In two such reports, Cu-loaded²⁷ and Ag-loaded MnO₂ catalysts³⁵ were prepared from the selective leaching of black mass obtained from discarded Zn–MnO₂ batteries. The black mass was treated with HCl (2 mol L⁻¹) at 80 °C for 4 h. The precipitate was filtered out and the filtrate (Mn-rich solution) was oxidised by NaClO solution at 45 °C for 6 h. The formed MnO₂ was phase-separated and washed several times with dilute HCl. While Cu loading over MnO₂ was performed by the wet impregnation method using copper nitrate salt, Ag was loaded by in situ reduction of Ag⁺ ions (from the silver nitrate salt) using NaBH₄ as the reducing agent (Fig. 1). The formed catalysts possessed a higher surface area than the black mass, which is beneficial for the catalytic removal of gaseous pollutants.

Fig. 1 Synthesis methodology for preparing Cu–MnO₂ (ref. 27) and Ag–MnO₂ catalysts.³⁵ EDX mapping of Cu–MnO₂ reproduced from ref. 27 with permission from Elsevier, copyright 2024. EDX mapping of Ag–MnO₂ reproduced with minor modification from ref. 35 with permission from The Royal Society of Chemistry, copyright 2024.

LIBs consist of a cathode, an anode, an electrolyte, a separator, and current collectors, along with packaging components. There are five major types of cathodes used in LIBs, namely lithium cobalt oxide (LiCoO₂), lithium iron phosphate (LiFePO₄), lithium manganese oxide (LiMn₂O₄), lithium aluminium cobalt nickel oxide (LiAl_xCo_yNi_1−x−yO₂), and lithium cobalt manganese nickel oxide (LiCo_xMn_yNi_1−x−yO₂).³ Since the cathodes of LIBs are composed of valuable metals, much research is focused on developing processes to leach out metals, extract Li as a soluble salt (with low Li purity), and synthesise transition metal-based functional materials.^36–38 In one such report, spent ternary LIBs were discharged in a 10% NaCl (aq.) solution for 24 h and manually disassembled to obtain the cathodes. The cathodes were calcined at 450 °C for 3 h to peel off active materials from the Al foils. The active cathode powder was treated with oxalic acid at 180 °C for 16 h in a Teflon-lined autoclave to extract Ni, Co, and Mn as insoluble oxalates. After phase separation and drying, the oxalate salt mixture was calcined at 550 °C for 3 h to yield NiCoMnO_x catalysts (Fig. 2).³⁷ In another study, the active cathode powder was treated with an H₂SO₄–H₂O₂ solution to leach Co, Ni, Mn and Li. While the obtained leachate was treated with KMnO₄ solution or ozone to form Co/Ni-doped MnO₂, the majority of Li, Co, and Ni remained in the solution phase.³⁶

Fig. 2 A schematic illustration of synthesis methodologies adopted for processing LIB's components into functional materials.^37,39,40 The PXRD pattern of synthesised carbon materials reproduced with minor modification from ref. 40 with permission from The Royal Society of Chemistry, copyright 2024.

While some researchers have developed novel processes to extract highly pure Li (as Li₂CO₃) along with transition metal oxides,^41,42 others have used the extracted Li₂CO₃ as a precursor for synthesising functional materials.^39,43,44 Qin and co-workers have extensively worked on extracting Li as pure Li₂CO₃ from LIBs' cathodes and subsequently using it as a precursor for synthesising Li₄SiO₄ sorbent for CO₂ capture. One such process involved discharging LIBs in a NaCl solution and manually disassembling them to obtain the cathodes. After treating the cathodes with NaOH solution to remove Al, the cathode material was placed in a leaching solution (H₂SO₄–H₂O₂, Na₂S₂O₈, or CH₃COOH–H₂O₂). The pH was adjusted with NaOH or NH₄OH to remove impurities and obtain a Li-rich solution. Na₂CO₃ was added to the solution phase at 95 °C to extract Li as a carbonate salt. The recovered Li₂CO₃, along with commercially available fumed silica, was mixed and calcined at 750 °C for 6 h to yield Li₄SiO₄ (Fig. 2).^39,44

Apart from the cathode of LIBs, the anode (graphitic carbon) after proper chemical treatment has been upcycled to yield functionalized carbon adsorbents.^40,45 Aravindan and co-workers developed reduced graphene oxide (rGO) from the graphitic anode by first oxidising graphite using a modified Hummer's method and then reducing the GO with hydrochloric acid and the outer metallic Al casing of the battery. The group also reported the template-free synthesis of porous carbon hollow spheres by one-step carbonization of the recovered polymer separators (polypropylene (PP) and polyethene (PE)) at elevated temperatures (Fig. 2).⁴⁰ Many methods have been reported to modulate the physiochemical properties of LIBs waste-derived materials by metal doping,⁴⁶ integration with porous substrates,^47,48 and acid etching.^49,50 Although these methods have not been discussed in detail in this section, their role in the catalytic properties of materials will be discussed in subsequent sections.

Gas capture and decontamination

In the literature, battery waste-derived functional materials have been primarily used as catalysts for the thermal oxidation of gaseous pollutants such as VOCs, NO_x, and CO_x. However, some recent studies have demonstrated that these materials could also be used as room-temperature adsorbents/catalysts for treating harmful acidic gases. In the subsequent subsections, these functional materials will be discussed in the context of capturing and removing various neutral and acidic gaseous pollutants.

Removal of VOCs

Volatile organic compounds (VOCs) are a diverse group of carbon-based chemicals that evaporate easily at ambient temperatures. They are found in common household products, including paints, solvents, cleaning agents, building materials, and furnishings. While many VOCs are only mildly irritating at low levels, some can have toxic effects on human health and the environment. Some VOCs are known carcinogens, while many are considered precursors of ozone, photochemical smog, and secondary aerosols. Low-temperature catalytic oxidation is considered a suitable method for eliminating VOCs from the environment. Supported noble metals and transition metal oxides are promising catalysts for the total oxidation of VOCs at low temperatures.⁵¹ Battery waste is rich in oxides of 3d-series transition metals and could be utilized for the purpose after suitable modification of the cathode material.

The black mass extracted from discarded Zn–MnO₂ alkaline batteries is rich in ZnO and MnO_x, which, after neutralization with mineral acids, was used for the thermal oxidation of VOCs at low-to-mid temperatures.^23,24,32 Black mass, after neutralization with H₂SO₄, catalysed the total oxidation of VOCs, including benzene, toluene, and o-xylene (collectively BTX) below 400 °C.²³ The type of acid used for neutralization played an important role in the catalytic activity. The catalytic activity followed this order: sulphuric acid > nitric acid > oxalic acid > hydrochloric acid > phosphoric acid > no acid, when these acids were used for neutralization. Neutralization with different acids yielded black mass with varying proportions of Mn and Fe (catalytically active metals) and surface area, where H₂SO₄-treated black mass possessed superior physicochemical properties.²⁴ The catalytic activity of acid-treated black mass was further improved by impregnating noble metals like Pd.³² After Pd loading, the black mass effectively catalysed benzene (T₉₀ ∼ 298 °C), toluene (T₉₀ ∼ 253 °C), and o-xylene (T₉₀ ∼ 239 °C) (note: T₉₀ is the temperature at which 90% conversion of VOC was observed). Without Pd, the T₉₀ value was 385, 331, and 377 °C for benzene, toluene, and o-xylene, respectively. Increased Pd loading enhanced the lattice oxygen mobility available for effective oxidation of aromatic VOCs. ZnO, from the acid-treated zinc rod (from spent alkaline batteries), was used as a support over which catalytically active Mn⁵² and Pd³³ sites were dispersed to oxidise VOCs. Increased MnO_x loading on ZnO improved the lattice oxygen mobility and strong acid-site density. BTX molecules adsorbed on the acidic sites reacted with the lattice oxygen to form CO₂ and H₂O. The formed O-vacancies were replenished by oxygen atoms formed by the dissociation of molecular oxygen (Fig. 3a).⁵² MnO_x/ZnO with 30 wt% Mn loading had a T₉₀ value of 411, 347, and 381 °C for benzene, toluene, and o-xylene, respectively. Contrarily, just 1.0 wt% of Pd loading over ZnO was sufficient for the total oxidation of BTX molecules at the same feed concentration, where the T₉₀ for benzene, toluene, and o-xylene was 389, 325, and 318 °C, respectively.³³ Increased metallic Pd loading favoured the catalytic process by improving the lattice oxygen mobility. On the electron-rich Pd sites, BTX molecules were adsorbed and subsequently reacted with lattice oxygen near the Pd–ZnO interface to form CO₂ and H₂O (Fig. 3b). Though only a minuscule amount of Pd is sufficient for catalytic process, the high cost of Pd compared to MnO_x could offset the overall catalytic gain.

Fig. 3 Proposed reaction mechanism for complete oxidation of BTX molecules over (a) MnO_x–ZnO⁵² (b) Pd–ZnO.³³

After comparing the T₉₀ values, it is evident that the thermal oxidation of benzene is more difficult than that of toluene or o-xylene as the lower ionization potential of methylated benzene derivatives makes their oxidation much easier than benzene.³² Another observation is that the oxidation of oxygenated VOCs like ethanol (T₅₀ ∼ 170 °C) over MnO_x catalyst is more favourable than that of aromatic VOCs like toluene (T₅₀ ∼ 280 °C). Ethanol is strongly chemisorbed on the MnO_x surface through the coordination of non-bonding oxygen electrons compared to the weak bonding observed for toluene through π-type C=C orbitals. Since strong chemisorption of ethanol lowers its activation energy, oxidation of ethanol occurs at a lower temperature than toluene.²⁹

Cathodes of all different types of LIBs have been upcycled to transition metal oxides for the thermal oxidation of VOCs. Artificially created Li, Co, and O vacancies in LiCoO₂ through partial Co³⁺ leaching and de-intercalation of Li⁺ ions by HNO₃-etching yielded catalysts possessing excellent oxidation efficiency for benzene. While pristine LiCoO₂ catalysed only 10% of benzene at 300 °C, 100% oxidation was observed for the acid-treated LiCoO₂ from synthetic air. The Li and O vacancies in LiCoO₂ facilitated benzene adsorption and activation, while the Co and O vacancies induced the formation of active surface oxygen species. The number of vacancies and subsequently the catalytic activity was further improved by increasing the acid-etching time. Also, the acidic solution rich in Li and Co could be further processed to recover valuable metals.⁵⁰ Mixed metal oxides, such as Co_3−xM_xO₄ (M = Li, Ni, Cu, Al, Mn), synthesised from LiCoO₂, catalysed toluene (T₉₀ ∼ 274 °C) better than the Co₃O₄ synthesised from nitrate salt (T₉₀ ∼ 282 °C). The superior activity of Co_3−xM_xO₄ was related to a higher surface area, better low-temperature reactivity, a higher density of weak acid sites, and a higher proportion of high-valent cations and lattice oxygen. While impurities like Cu and Mn in the catalyst improved the activity, Li negatively affected it.³⁸ Several other mixed metal oxides like CoMnNiO_x for the oxidation of propane,⁵³ MnO_x (CoMn₂O₄–NiCo₂O₄–Cu_1.2Mn_1.8O₄–Mn₂O₃) for the oxidation of 1-methoxy-2-propanol,⁴⁷ CuNi_0.5Mn_1.5O₄ for the oxidation of toluene, ethylbenzene, and formaldehyde,⁵⁴ and Mn₅O₈/CeO₂, CuNi_0.5Mn_1.5O₄/CeO₂, and Co₃O₄/CeO₂ for the oxidation of 2-ethoxy ethyl acetate,⁵⁵ have shown the potential of LIBs-derived catalysts in VOC elimination. In all these catalytic systems, a higher surface area, abundant high-valent transition metal sites, and a higher proportion of adsorbed oxygen species in catalysts determined their VOC oxidation activity.

On numerous occasions, LIBs-derived manganese oxides (MnO_x), especially MnO₂, have been reported for VOC oxidation. It is noteworthy that high-valent Mn-sites in MnO_x catalysts (in MnO₂) catalyse VOCs better than the low-valent Mn-sites (in Mn₂O₃ and Mn₃O₄).^28,56 Even the presence of different phases of MnO₂ and dopants could alter the catalytic activity of MnO_x. In one such study, Co/Ni-doped α-MnO₂ and β-MnO₂ catalysts were synthesised by the oxidative precipitation of Mn-rich leachate using KMnO₄ and ozone, respectively. While doped catalysts performed better than the pure MnO₂ catalysts, doped α-MnO₂ catalysed the oxidation of toluene and formaldehyde better than doped β-MnO₂.³⁶ α-MnO₂ with a [2 × 2] tunnel structure is more active than β-MnO₂ with a [1 × 1] tunnel structure, as the [2 × 2] tunnel structure could facilitate the absorption and diffusion of VOC molecules towards active sites.⁵⁷ An appropriate proportion of transition metal dopants like Co and Ni in MnO₂ could improve the catalytic activity by increasing the concentration and mobility of lattice oxygen. Similar observations have been made with Cu-doped α-MnO₂ during the oxidation of toluene and chlorobenzene. Cu-sites in α-MnO₂ favoured the catalytic process by increasing the surface oxygen species and active oxygen species through the Cu²⁺/Cu⁺ redox cycle.⁵⁸ Even the morphologies of catalysts could influence their catalytic activity, as was the case with morphologically different MnO₂ catalysts, MnO₂-CM (microspheres), MnO₂-HM (stacked nanorods), and MnO₂/SBA-15 (microspheres), synthesised by hydrothermal, co-precipitation, and impregnation methods, respectively, for the oxidation of toluene. MnO₂-HM showed 100% conversion of toluene even with the lowest surface area due to its better redox ability and the large presence of adsorbed oxygen species at a low synthesis temperature of 140 °C.⁴⁸

On rare occasions, spent ternary LIBs have been processed to synthesise perovskite catalysts, LaMn_1−xB_xO₃ (B = Co, Ni, Cu, Al) for the thermal oxidation of toluene⁵⁹ and SmMnO₃ and SmCoO₃ for propylene glycol methyl ether.⁶⁰ The LIBs-derived perovskites performed better than those synthesised from commercial salts, as the former has a higher surface area, Mn⁴⁺/Mn³⁺ ratio (or Co³⁺/Co²⁺ ratio for Co-perovskite), and lattice oxygen species. The partial substitution of Mn/Co sites with Ni, Co, Al, and Cu ions improved catalytic performance. However, the presence of Li as an impurity in the catalyst severely inhibited the activity.

Plasma-assisted catalytic oxidation of VOCs is another method that has been studied using battery waste-derived catalysts like Mn₂O₃ (ref. 26) and Mn₂O₃/Al₂O₃ (ref. 61) with BTX pollutants. Though the method effectively catalysed a quantitative amount of benzene and toluene, the oxidation of o-xylene was below 75% even under the best experimental conditions. The catalytic activity was significantly affected by synthesis conditions, including the type of acid, acid concentration, black mass-to-acid volume ratio, calcination temperature, and Mn content over the alumina substrate.^26,61 Other than thermal oxidation, photocatalytic oxidation of toluene (∼50 ppm) has been performed over TiO₂-modified black mass (from Zn–MnO₂ alkaline battery).^31,62 The composite photo-oxidized 100% of toluene within 3 h of irradiation, better than TiO₂ alone, black mass alone, or even the TiO₂–ZnMn oxide composite synthesised from black mass leachate. The TiO₂–black mass composite, with higher photon absorption efficiency, better charge transfer properties, and low charge-recombination rate, effectively absorbed photons and generated superoxide and hydroxide radicals for rapid photo-oxidation of toluene.⁶² Even the calcination temperature during the synthesis of TiO₂–black mass composite played a major role in the photocatalytic efficiency. The photo-catalyst synthesised at a lower calcination temperature (200 °C) showed the highest catalytic activity due to the highest surface area, highest photon absorption property, narrowest band gap, and lowest charge recombination efficiency.³¹ Thus, low-concentration VOC sources are effectively treated by photo-oxidation using TiO₂–black mass composites.

Removal of CO and NO

Carbon monoxide (CO) is a highly poisonous gas produced from the incomplete combustion of organic matter. CO must be removed from tail gases, and for that, thermal oxidation of CO to CO₂ is a viable method. Battery waste-derived catalysts could efficiently oxidise CO to CO₂ at low-to-mid temperatures. Zn–MnO₂ battery-derived Cu–MnO₂ (ref. 63) and Ag–MnO₂ (ref. 35) completely oxidised CO at 120 and 150 °C, respectively, performing better than MnO₂ (240 °C). The inclusion of Ag and Cu in MnO₂ played a synergistic role and improved the catalytic activity. Importantly, MnO₂ with higher Cu loading performed better by providing a large surface area, more surface-active oxygen species and oxygen vacancies, and a higher proportion of Mn⁴⁺ and Cu²⁺ ions for catalysis.⁶³ CoFe₂O₄–LIB (CoFe₂O₄–Co₃O₄), derived from LIBs' cathode, has been used for CO oxidation. The composite oxidised 98% of CO at 300 °C, better than pure CoFe₂O₄ (60% CO oxidation at the same temperature). Detailed spectroscopic investigations suggested that both CoFe₂O₄ and Co₃O₄ were essential for CO oxidation, where CO was oxidised to CO₂ by interacting with the adsorbed oxygen species and lattice oxygen over CoFe₂O₄ and Co₃O₄, respectively (Fig. 4).⁴¹

Fig. 4 The role of CoFe₂O₄–LIBs catalyst in CO oxidation involves the interaction of CO with (a) adsorbed oxygen species over CoFe₂O₄ and (b) lattice oxygen over Co₃O₄.⁴¹

Nitrogen oxides (NO_x), emitted from thermal power plants and incinerators, are some of the toxic gaseous pollutants responsible for acid rain and photochemical smog. Low-temperature NH₃-selective catalytic reduction (NH₃-SCR) technology is promising in reducing the NO_x concentration from stationary sources due to its high de-nitration efficiency. The process involves NH₃-driven reduction of NO to N₂ at low temperatures over a suitable catalyst. Though V₂O₅–WO₃/TiO₂ (an industrial catalyst) is used for the process, the high cost of V and W, V toxicity, low SO₂ resistance, and poor low-temperature activity make it essential to explore other economical and effective catalysts. Mn-based catalysts are inexpensive and perform well even at low temperatures.⁶⁴ Spent alkaline Zn–MnO₂ batteries are a good source for extracting necessary Mn for synthesising catalysts. Chang and co-workers estimated that CO₂ emission during carbothermal extraction of Mn from spent batteries was 1.0 kg per kg of black mass, which was ∼15% of the CO₂ emitted during traditional Mn manufacturing (6.7 kg per kg Mn). Moreover, the recovered Mn cost is much lower than the commercial Mn salts. Thus, processing primary batteries for Mn seems to be an economical and environmentally benign approach. The battery-derived Mn/TiO₂ catalyst was found superior to commercial salt-based Mn/TiO₂. The superior catalytic activity and high N₂/N₂O selectivity of Mn/TiO₂ at 160–200 °C were ascribed to a higher Mn⁴⁺ proportion and favourable presence of Fe³⁺ impurity, which improved the oxidation ability of battery-derived Mn/TiO₂. However, certain impurities like carbon and Zn in treated black mass could reduce the activity of Mn/TiO₂ by decreasing the Mn⁴⁺ concentration and acidity of the catalytic surface. Thus, caution must be taken during the processing of black mass for Mn extraction using the carbothermal method.⁶⁵

After Li recovery from LIBs cathode using oxalic acid, the residual precipitate was upcycled to yield NiCoMnO_x catalyst for NO_x reduction. NiCoMnO_x (Co₃O₄–NiCo₂O₄–MnCo₂O_4.5–NiO) catalysed 90% of NO_x in a broad temperature range of 110–230 °C, better than the catalyst synthesised with metal salts. The LIBs-derived catalyst possessed a higher surface area and abundant oxygen defects, which favoured the adsorption of reactant gases, i.e., NO and NH₃ (Fig. 5). Though the LIBs-derived catalyst performed well in the presence of moisture, SO₂ in the feed gas severely hampered its catalytic activity by reacting with the adsorbed NH₃ to form ammonium sulphate and bisulphate species over the catalyst surface.³⁷ Polyethylene glycol (PEG), a water-soluble polymer, was used as a templating agent during the LIBs processing to synthesise NiCoMnO_x catalysts with modulated physicochemical properties. The NiCoMnO_x–PEG catalyst possessed a higher surface area and a higher proportion of Co³⁺, Mn⁴⁺, and adsorbed oxygen species, which favoured the oxidation of NO to NO₂ (fast SCR reaction). Though NiCoMnO_x catalysts synthesised with or without PEG catalysed 90% of NO in a similar temperature window of 80–230 °C, NiCoMnO_x–PEG performed better in the presence of H₂O and SO₂ in the feed gas. A higher SO₂ tolerance of NiCoMnO_x catalyst was due to the presence of more acidic sites over the surface, which delayed SO₂ adsorption and subsequent conversion to sulphate species.⁶⁶ Zhang and co-workers modulated the physiochemical properties of LiMn₂O₄ (simulated cathode of LIBs) by exchanging some of the Li⁺ ions with protons (acetic acid treatment) and/or by surface loading with a small amount of V₂O₅. While the inclusion of protons increased the surface acidity, V₂O₅ regulated the redox behaviour and increased the number of acidic sites. The Li_xH_1−xMn₂O₄–V₂O₅ (dual-modulated) catalyst showed the best combination of NO reduction activity and N₂ selectivity than singly modulated catalysts due to improved NH₃ adsorption, superior nitrite decomposition, and reduced surface nitrate species formation.⁴⁹ These studies have highlighted that after suitable modifications, cathodes of waste LIBs could be upcycled as efficient NH₃-SCR catalysts.

Fig. 5 Proposed NH₃-SCR mechanism for NO conversion to N₂ over waste LIBs-derived NiCoMnO_x catalyst.³⁷

Removal of acidic gases

Although carbon dioxide (CO₂) is a weakly acidic gas, it is considered an important greenhouse gas responsible for rising global temperatures and climate change. Carbon capture is a leading technology that relies on porous materials for preferential sequestration of CO₂ from flue gas or even air.⁶⁷ Carbon-based porous materials like reduced graphene oxide (rGO, surface area ∼ 374 m² g⁻¹) and porous carbon hollow spheres (surface area ∼ 402 m² g⁻¹) have been developed from LIBs' anodes (graphitic carbon) and polymer separators, respectively. These materials have shown potential in capturing CO₂ gas at 25 °C and 40 bar, with adsorption capacities reaching 61.2 and 170.3 cm³ g⁻¹ for rGO and carbon spheres, respectively.⁴⁰ Although these adsorbents store a large volume of CO₂ at 40 bar, for practical purposes, the material must be functional at low CO₂ pressures. A K₂CO₃-impregnated carbon residue has been demonstrated as a reusable adsorbent for capturing CO₂ from indoor air (1000–2000 ppm). In moist conditions, hydrated K₂CO₃ reacted with CO₂ to form bicarbonates. Through this reaction, the material adsorbed ∼9.7 wt% of CO₂ at 25 °C. Regeneration of the adsorbent requires heating to 150 °C and was regenerable for four cycles.⁴⁵

High-temperature CO₂ chemisorption over alkali ceramics is a well-established approach where CO₂ is chemisorbed as a carbonate salt of an alkali metal. Lithium orthosilicate (Li₄SiO₄) is a lithium ceramic that has been explored as a CO₂ captor and possesses high capacity and regenerability.⁶⁸ More often, different types of cathodes from spent LIBs have been used to recover Li as Li₂CO₃, which was further utilized as a Li precursor for synthesising Li₄SiO₄. In one such strategy, CO₂ sequestration was done by using CO₂ as a leaching agent to extract Li out of the spent LiFePO₄ cathode as highly pure Li₂CO₃. Moreover, the FePO₄ residue was converted to Na₄Fe₃(PO₄)₂P₂O₇ (a cathode for NIBs). This approach effectively fixed ∼120 kg of CO₂ for every ton of recycled LiFePO₄.⁴² A more direct approach involved using extracted Li₂CO₃ and a Si source (commercial silica or rice husk ash) to synthesise Li₄SiO₄ by a solid-state route. Through this, the cost of producing Li₄SiO₄ was reduced by 25–95% compared to the conventional route involving commercial precursors.^39,43,44,69 Different types of cathodes, carbon sources for pyrolysis of cathodes,³⁹ and leaching medium⁴³ affected the composition of Li₂CO₃, further influencing the physicochemical properties of Li₄SiO₄. The best CO₂ uptake was recorded for LiFePO₄ battery-derived Li₄SiO₄, where the material possessed a capacity of 270–280 mg g⁻¹ for 80 cycles in a 15 vol% CO₂ feed.⁴³ Also, waste (battery and rice husk)-derived Li₄SiO₄ was found to be a better CO₂ captor than those synthesised with commercial precursors,⁶⁹ further highlighting the importance of battery waste-derived materials.

Spent alkaline Zn–MnO₂ batteries have been upcycled as adsorbents/catalysts for treating highly acidic gases like H₂S, SO₂, and NO₂. Zn_xMn_3−xO₄ and ZnO synthesised from the leachate of Zn–MnO₂ batteries were used for treating H₂S gas at 400 °C. These transition metal oxides successfully removed H₂S from simplified gas mixtures and real syngas by reacting with H₂S to form α-MnS, γ-MnS, ZnS, elemental sulphur, polysulfide, and MnSO₄. The binary metal oxide with a high Mn/Zn ratio possessed superior catalytic activity and favoured the oxidation of sulphide to elemental sulphur and sulphate.³⁰ Since black mass derived from Zn–MnO₂ batteries is highly alkaline and has active ZnO, MnO₂, and ZnMn₂O₄ phases, it was possible to use the raw black mass directly for H₂S removal at 20 °C and high humidity. The black mass possessed an adsorption capacity of 60 mg g⁻¹ for five cycles. Over the black mass, H₂S molecules dissociated to sulphide and further oxidised by H₂O and molecular O₂ to sulphur and sulphate. Theoretical calculations confirmed that ZnO was more reactive than MnO₂, and black mass derived from a fully discharged battery is expected to show even higher adsorption capacity.²² In a separate study, untreated black mass from Zn–MnO₂ batteries of different brands was used as catalysts for treating 100 ppm of SO₂ and NO₂. These black mass samples have different phase composition, surface area, and alkalinity, which influenced their SO₂ uptake capacity. For three cycles, the best material exhibited an average SO₂ and NO₂ adsorption capacity of 30.2 and 13.8 mg g⁻¹, respectively, at 20 °C and high humidity. Further performance improvement was achieved by integrating black mass with calcium alginate to form hydrogel beads. The black mass catalysed SO₂ to sulphate and NO₂ to nitrite and nitrate ions.²¹ These studies highlighted the potential of black mass in treating a wide range of acidic gases at room temperature. Unlike CO₂ captors like Li₄SiO₄, which require high energy input for both material synthesis and CO₂ chemisorption, direct use of black mass (ZnO–MnO₂–KOH) in treating acidic gases at low temperatures should be promoted for sustainable processing and upcycling of spent batteries (Fig. 6).

Fig. 6 An illustration showing energy-intensive and energy-efficient upcycling of battery waste for acidic gas removal.

Challenges and prospects

Waste LIBs and alkaline batteries are processed to develop functional materials for air decontamination applications. The synthesised materials serve as catalysts for the thermal oxidation of VOCs to CO₂ and H₂O, NH₃-SCR catalysts for reducing NO_x to N₂, and room-temperature adsorbents/catalysts for eliminating acidic gases like CO₂, SO₂, NO₂, and H₂S. Many of the battery-derived catalysts have shown higher catalytic performance than those synthesised with commercial salts. Methods have been developed to sequestrate CO₂ either by adsorbing or by converting it into carbonates. Additionally, CO₂ has been employed as a leaching agent to extract pure and high-valued Li₂CO₃ from waste LiFePO₄ batteries. To a greater extent, the main objectives of processing battery waste and developing functional materials for gas capture and removal applications have been fulfilled by the current literature. However, challenges persist for its translation from lab to industry. The biggest challenge in waste battery processing is the variation in the types of batteries available in the market. There is no one method which could be applied to all kinds of battery waste. This underscores the importance of consumer awareness in segregating battery waste at its source before collection by relevant authorities.

The second challenge associated with battery waste processing is the excessive consumption of concentrated mineral acids like sulphuric or nitric acid for the leaching of desired or undesired metals from the battery's cathodes. This practice must be discouraged as mineral acids are toxic, require large energy input during production, and upon unchecked disposal are catastrophic to the environment.⁷⁰ In such a scenario, the use of hydrometallurgical methods like bioleaching of metals using sulphuric acid-producing bacteria like Acidithiobacillus thiooxidans must be encouraged, which could reduce the volume of secondary waste generated from the cathode processing.^28,29 Another issue faced during the upcycling of battery waste to functional materials is excessive energy consumption during the calcination process. Some studies have highlighted that materials synthesised at lower temperatures are more efficient than those synthesised at higher temperatures. An energy optimization during the synthesis process could greatly reduce the overall cost of upcycling and CO₂ emissions associated with thermal energy consumption. Another approach is to upcycle battery waste with minimum steps to target a desired gas pollutant. It was observed that the black mass from alkaline batteries could be directly used for capturing H₂S at room temperature. Thus, the need to upcycle the black mass to metal oxide catalysts for mid-temperature removal of H₂S could be avoided. Moreover, many energy-intensive processes involved in gas treatment, like high-temperature CO₂ chemisorption over alkali ceramics, must be discouraged. Room-temperature sequestration of CO₂ over porous carbonaceous materials derived from polymer separators is a viable option for carbon capture, which should be explored more and extended to a larger scale after a systematic cost-to-performance evaluation. Though catalytic oxidation of aromatic VOCs is possible only at mid-temperatures, other VOCs like formaldehyde can be degraded even at room temperature.⁷¹ Even photocatalytic oxidation using solar radiation is a feasible option for eliminating aromatic hydrocarbons like toluene over battery-derived catalysts. Implementing these energy-efficient strategies could reduce the generation of secondary waste, overconsumption of energy, and associated CO₂ emissions during the upcycling of battery waste. All these strategies are scarcely reported in the literature and need detailed investigation before their implementation in real conditions.

The urgent need to transition from fossil fuel to a renewable energy-driven economy is fuelling the growth of energy storage systems. Much of the research is concentrated on developing LIBs that could hold sufficiently large amounts of energy and are safe during operation. While research and development in battery manufacturing is a must to achieve the green economy targets, attention should also be given to the processing of battery waste. So far, a large volume of used batteries are either stored or disposed of in landfills. The situation for disposal of primary batteries is even worse as these batteries are consumed in billions with poor mechanisms for waste battery collection and processing in most of the countries. Though it is doubtful that the extraction of valuable metals like Li, Co, Ni, and Cu in LIBs could offset the entire cost of recycling, upcycling battery waste as functional materials and using them for gas pollutant removal could be an economical measure towards a cleaner environment. In the future, millions of tons of LIBs and Zn–MnO₂ batteries are expected to be available for processing. Public and private entities must act responsibly in tackling this environmental issue by educating the public about battery segregation and recycling. The government must incentivise the entities focused on recycling and repurposing of battery waste. “A better segregation strategy is the golden step towards processing any waste”. Soon, more efficient and safer batteries like NIBs are expected to be rolled out in the market, which will further increase the volume of battery waste. Unlike LIBs, NIB's recycling cost will be even harder to offset as these are made with inexpensive metals like Na and Mn. Though some of the recent works have highlighted the potential of NIBs' cathode (Na–Mn oxides) in cleaning acidic gases in ambient conditions,^72–74 more efforts are needed in identifying and developing functional materials capable of degrading harmful organic/inorganic gaseous pollutants in ambient conditions. The future of upcycling battery waste is bright as the research is moving in the right direction and targeting the most important issues of rising battery waste and air pollution.

Conclusions

The upcycling of discarded primary alkaline batteries and rechargeable LIBs to functional materials for harmful gas removal represents a promising approach to addressing the rising concentration of gaseous pollutants in the atmosphere. Through innovative approaches, electrodes, electrolytes, and polymer separators of battery waste could be processed into various functional materials, including metal oxides, composites, and carbonaceous materials. The synthesised materials have been effectively utilized as catalysts for thermal oxidation of VOCs and CO, NH₃-SCR catalysts for NO_x reduction, and room-temperature adsorbents/catalysts for capturing and mineralizing acidic gases. In many cases, the battery waste-derived materials were found more efficient than those synthesised by metal salts for a desired application. Numerous strategies, like metal-doping, integration with porous substrates, and acid/base-etching have been implemented to further improve the performance of these materials. These research findings have shown immense potential in repurposing battery waste streams for creating high-performing materials capable of effectively removing harmful gases from various sources. As research in this field continues to advance, it holds the promise of not only improving air quality but also promoting the circular economy by transforming waste into valuable resources.

Author contributions

NKG is responsible for the literature survey and manuscript writing and reviewing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

NKG would like to acknowledge Prof. Srungarpu N. Achary, Scientific Officer at the Solid State Chemistry Division, Bhabha Atomic Research Centre (BARC), Mumbai, India, for valuable scientific discussions.

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