Cathode regeneration and upcycling of spent LIBs: toward sustainability

Xiang Xiao ^a, Li Wang *^a, Yingqiang Wu ^a, Youzhi Song ^a, Zonghai Chen *^b and Xiangming He *^a
aInstitute of Nuclear and New Energy Technology, Tsinghua University, Beijing, 100084, China. E-mail: wang-l@tsinghua.edu.cn; hexm@tsinghua.edu.cn
^bChemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, IL 60439, USA. E-mail: zonghai.chen@anl.gov

First published on 2nd June 2023

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Broader context With the continuous development of EVs and renewable energy, LIBs are widely applied in industry and daily life. Large-scale production and extensive applications of LIBs have brought serious concerns in recent years, including not only the fluctuations in prices of raw materials but also the doubt about the contribution to energy/emission reduction of the LIBs in their lifecycles, as well as the urgent concern regarding disposal of spent LIBs. These problems can be alleviated or potentially solved via efficient cathode regeneration technologies. However, there have been debates and controversies within the academic community about the optimal route/method for LIB recycling. Instead of crudely extracting metal composition from crushed battery waste through a destructive energy-intensive metallurgical process, degraded cathode materials can be easily recovered and directly used for electrode preparation and cell/pack manufacturing. The circular routes of LIBs using regeneration methods are greatly shortened with increased recovery rate, reasonable profits and higher energy utilization efficiency. Considering the frequent technological progress and rapid product iteration of LIBs, the cathode upcycling method is showing excellent flexibility to adapt to the upgrading of battery chemistry and boosting the LIB industry to step towards a circular economy from a resource-based economy. Besides, it should be mentioned that the complex pretreatment procedures need to be improved urgently. From the holistic perspective of the complete life cycle of LIBs, sustainability should be considered throughout the battery/pack design, raw material acquisition, materials synthesis, manufacturing, serving, retiring, and recycling processes of LIBs.

1. Introduction

The global energy infrastructure is transforming toward being green or sustainable, shifting from traditional fossil fuels to renewable energy sources. LIBs play an essential role in this green revolution in two ways: (1) as energy storage systems (EESs) to aid the grid-level practical operations of solar, wind, and hydroelectric power stations, which have issues associated with intermittent power supply;¹ (2) as energy sources for electrical vehicles (EVs) freeing us from the dependence on fossil fuels.² Driven by ever-increasing energy demands and the optimistic anticipation of the universal uptake of EVs and renewable energy sources, there has been tremendous growth in LIB production over the last decade (Fig. 1(A)³). However, with the volume production and increase in the use of LIBs, issues have arisen regarding the availability and price fluctuations of raw materials and the disposal of end-of-life (EOL) LIBs, becoming a reality that hinders the continuous and sustainable growth of the green economy.

The contradiction between the manufacturing costs of LIBs and soaring prices of raw materials

Over the last decade, with the technical advances made in mass production processes and the expansion in their production scale, the price of LIBs has decreased by 85% from 917 $ per kW h in 2010 to 137 $ per kW h in 2020 (Fig. 1(B)). However, according to BloombergNEF,⁴ the cost of LIBs only fell to 132 $ per kW h in 2021, a smaller price decrease than expected. For comparison, the average yearly reduction rate in the price of LIBs has been 19% for the last decade. Based on this past trend, the price of LIBs is expected to fall to 58 $ per kW h in 2030, representing a 58% reduction in price compared to 2020. However, the prices of raw materials for producing cathodes for LIBs, especially lithium (essential for producing all LIB cathode materials) and cobalt (used to produce layered oxide cathode materials), have sharply increased since 2019 (Fig. 1(C)⁵). It should be noted that the cost of cathode materials accounts for more than half of the total costs of all components of LIBs (Fig. 1(D)⁶). According to data from Trading Economics,⁵ the prices of lithium salts, including hydroxide and carbonate, have skyrocketed since 2019 with an appalling increase of ∼600%. The price of cobalt has also fluctuated since 2019 and reached a peak in the first quarter of 2022. In any case, the exorbitant costs of the essential raw materials that are used to produce cathodes are detrimental to the LIB industries and markets and hinder the deeper penetration of EVs and promotion of EES, thus slowing the pace of the green revolution of the global energy infrastructure.

Disposal of retired LIBs

LIB cells or packs will be retired (i.e., entering their EOL stage) when their capability to store energy is significantly compromised below their initial rating value and unsuitable for continued operation in EVs or ESS applications. Usually, this degradation in capacity occurs after hundreds of complete charging/discharging cycles or an 8 year-calendar life. Furthermore, with the remarkable progress made over the last decade in battery science by researchers and engineers, battery products have been continuously upgraded, in terms of energy, power density, and safety, every couple of years with different cathode materials. Along with the vast and ever-increasing production of LIBs, the volume of retired LIBs is also considerable, with a 20% annual growth rate (Fig. 1(E)³). If these spent LIBs are not correctly handled, battery waste with heavy metals, fluorochemicals, and organic electrolytes will easily damage the soil and pollute groundwater, presenting a major environmental issue.

It is well accepted that under the worldwide theme of sustainability, LIB industries will not survive without recycling. The materials in the cathodes, such as nickel, cobalt, and lithium, are of value and highly recyclable.^7,8 Not only does recycling address the pollution concerns from EOL battery waste, but it also eliminates the need to extract raw materials from minerals, which is associated with the generation of carbon emissions and other issues (such as energy consumption, greenhouse gas (GHG) emissions, labor, and human rights). It should be noted that carbon emissions from the production phase of a battery account for 36% of the total emissions of the life cycle of LIBs,⁹ mainly due to the mining and processing of nickel, cobalt, and lithium. According to estimations by the European Environment Agency, for every 1 kW h LiNiCoMnO₂(NCM)-based LIB produced, 63 kg of carbon emissions are generated.⁹ Effective recycling of LIBs can reduce carbon emissions from material resource extraction by 70%. Besides, by recycling EOL battery waste to extract needed elements, a secondary domestic supply chain independent of the traditional mining supply chain could be secured that would reduce socio-economical risks, naturally free from uncontrollable price fluctuation, and would ensure a dependable supply of raw materials. The industrial and academic communities have identified this immense potential and urgency for recycling retired LIBs, with enormous efforts being made to achieve this aim over the past decade (Fig. 1(F)). However, the large-scale recycling of batteries still presents a challenge due to the high capital input required, low profitability, and crucial technological advances that need to be made.

Herein, the technologies and advances in cathode recycling methods are summarized and analyzed, covering indirect metallurgy and direct regeneration. The advantages and disadvantages of these methods in terms of environmental issues and finances are discussed in detail. The failure and degradation mechanisms of spent cathode materials are also briefly analyzed to justify the feasibility and development trends of direct regeneration methods. Cathode upcycling is highlighted as the most promising and optimal recycling method to improve the sustainability of LIBs.

2. Comparison of cathode recycling methods

Fig. 2 shows a flow diagram of the complete life cycle of LIBs from raw material acquisition (ore mining) to the preparation of active electrode materials, cell/pack manufacturing, serving in EVs or EES, the EOL phase (spent LIBs), and their eventual destination as landfill waste or recycling as materials that have added value. Although LIBs are extensively used in green energy applications to navigate away from the use of non-renewable fossil energy resources, the manufacturing process of LIBs is traditional, starting from ore mining to acquire the requisite elements (such as Li, Mn, Ni, Co, etc.), followed by a refinement process employing metallurgy technology, to the synthesis of active electrode materials through high temperature sintering, and finally cell/pack manufacturing powered using fossil energy. With the widespread application and fast-growing production of LIBs (Fig. 1(B)), the energy/water consumption and GHG emissions produced from the traditional processes used to manufacture LIBs have raised public concern.^10,11

With the malfunction or loss of the original capacity of LIBs, the massive number of EOL LIBs need to be handled cautiously. Considering the valuable metal content of LIB cathodes (Fig. 1(F)) and its impact on the environment if directly disposed of, cathode recycling and/or regeneration is a more appropriate and efficient disposal method than sending these materials to landfill. Ideally, the designed recycling/regeneration process should allow materials to be collected or mineral elements or active materials to be recovered for manufacturing new LIBs, thus partially freeing the LIB manufacturing industry from the above-mentioned environmentally unfriendly and energy/water-consuming processes of the traditional mining-refinement-manufacturing path. However, the state-of-the-art technologies for recycling LIBs cannot be applied in large-scale commercial settings, handle the volume of retired LIBs, or provide enough raw materials for manufacturing. The LIB recycling industry faces issues such as high energy consumption, pollution, the requirement for significant capital investment, low efficiency, and questionable profitability. There have been debates and controversies within the academic community about the optimal routes/methods for LIB recycling.^12–21

Cathode recycling techniques can be classified into indirect recycling and direct regeneration.^15,22–24 The former is aimed at salvaging chemical elements, such as lithium, nickel, and cobalt, using pyrometallurgy or hydrometallurgy;^25–30 the latter is dedicated to restoring the chemical composition, crystalline structure, and morphology of the cathode particles without particle destruction during processing.^31–33 The primary routes for the processes involved in these two recycling methods are presented in Fig. 2. First, spent LIBs need to be pretreated (deactivation, battery disassembly and component separation) before recycling processes. However, the pretreatment steps and requirements for indirect recycling and direct regeneration techniques are different. As for indirect recycling, the spent LIBs are simply crushed and sieved to obtain black powders, ready for the indiscriminate metallurgical element extracting processes. In contrast, for direct regeneration, degraded cathode active materials should be obtained without damaging the original crystal structures and chemical composition. Besides, impurities from binders, conductive agents, electrolytes, and battery-cycling accumulated by-products should also be removed before the actual regeneration processes. Before that happens, cathodes (with current collectors) need to be manually separated from other cell components (including anodes, separators, and battery cases), then rinsed with organic solvents to remove side products from parasitic reactions in the cathode electrolyte interphase (CEI) and electrolyte residual.^34,35 Then, active cathode materials can be peeled off from the current collectors via soaking in solvents under stirring and ultrasonication. In addition, in order to remove conductive carbon and polymer binders, the precipitated spent cathode powders are further annealed at high temperatures.^35–37 It is delightful to notice a novel advance in materials separation from Huang and coworkers, who proposed a transient heating (1500 °C, 1s) approach for electrode material separation with 97% recovery ratio of active materials and nearly 100% of metal foils.³⁸ Meanwhile, binder network and conductive carbon can be effectively decomposed into gases under the dry high-temperature heating pretreatment beyond simple soaking by organic solvents, which is not practical and sustainable for scalable adoption. Flow charts of the pretreatment procedures of both recycling methods are shown in Fig. 3. It is obvious that the direct regeneration route involves more complicated and demanding processes, such as effective battery disassembly, cautious active material peeling, usage of costly solvents, and removal of impurities, thus causing difficult hindrance in the scalable application of direct regeneration with compromised profitability and full-cycle efficiency.

After pretreatment, slag or crushed black powder from spent LIBs is processed via pyrometallurgical, hydrometallurgical, or combined pyro- and hydrometallurgical methods, which completely disintegrate the structure of the cathode materials and allow salvaging of the valuable components. Usually, the recycled components require further refinement or composition adjustment before being used as raw materials in the manufacture of LIBs. To a certain extent, the indirect recycling route replaces the ore mining process. LIB re-manufacturing processes based on indirect recycling involve energy-intensive and environmentally harmful routes, such as high-temperature treatment, leaching with toxic acids, refinement of metals, cathode sintering preparation, and cell/pack manufacturing. The flow path of indirect recycling is still time-consuming and complicated and is associated with many environmental issues, meaning that it does not meet the requirements of being a revolutionary practical recycling technology associated with minimum energy/water consumption, the low release of polluting gases or waste residues, and highly-efficient short procedures. In the direct regeneration route, the repaired/upcycled cathode materials are collected and regenerated without particle-level destruction and can be directly used for cell manufacturing. The procedures are significantly simplified in this circular regeneration process, leading to increased efficiency and sustainability.

Indirect recycling and direct regeneration routes have been compared regarding their applicability, pretreatment requirements, scalability, environmental friendliness, economic benefits, and recovery rate, as shown in Fig. 4(A). The metallurgy process is lengthy and associated with adverse environmental burden. Moreover, another bottleneck in promoting indirect recycling technology is its applicability to various cathode chemistries. Stationary energy storage systems are usually extremely cost-sensitive with less concern about energy density. Therefore, LiFeO₄ and LiMn₂O₄ (LMO) cathodes with less valuable metal constitutions are widely adopted, and indirect metallurgy methods might not be suitable for recycling these cathode materials due to inadequate profit. It needs to be mentioned that the currently reported techno-economic analyses upon indirect/direct recycling routes might be inaccurate, because most data (e.g., energy consumption, cost, carbon footprint, and profit) are derived from lab-scale assumptions rather than real-life scalable industry. Furthermore, the volatile prices of raw materials also increase the difficulty in profit evaluation. Though, it still can be concluded from Fig. 4(B–E) that direct regeneration is a more environmentally and economically viable route than indirect recycling,¹ as although the indirect recycling method has advantages in terms of fewer pretreatment requirements and better potential for scalability, the direct regeneration route is still more likely to be the optimal method for EOL LIB recycling given its environmental and financial sustainability.

3. Direct regeneration methods

Direct regeneration technologies are showing promising potential due to their straightforward repairing or upcycling processes of spent cathode materials without disintegrating their original crystal structure and chemical composition. Before elaborating on the technical details of direct regeneration methods, the scientific fundamentals of the LIB failure/degradation mechanism need to be briefly analyzed to justify the feasibility and development trend of the direct regeneration methods. As shown in Fig. 5, the degradation mechanisms of commercial LIB cathodes vary with the cathode chemistries and crystal structures. For layered oxide cathodes (such as LCO and NCM), Li-loss, severe particle cracking, transition metal (TM) dissolution, and inactive impurity phases (spinel and rock-salts) are frequently reported as the degradation reasons for decreased capacity, inferior rate performance, and poor cycling stability.^42,43 Besides, the failure mechanisms for olivine cathodes can be summarized as Li-deficiency, morphology degradation, and irreversible phase transformation.^44,45 It should be emphasized that although the spent batteries lose a significant portion of their original discharging/charging capacity or fail to serve in specific circumstances, cathode materials still preserve most of their dedicated crystal structure and chemical constitution, and these features are desired for high performance cathodes and are originally obtained through energy-intensive sintering processes. It has been reported in many investigations that the defects causing battery degradation can be effectively repaired and the spent cathode materials can be upcycled via a well-designed process.³¹ Furthermore, it should also be noted that direct regeneration methods must be adjusted according to the various failure/degradation mechanisms for different types of cathode materials under diverse working conditions. For brief classification and preliminary material defect diagnostics, cathode types should be first determined in the pretreatment stage, considering the distinct degraded behaviors/reasons for various cathode materials. Cell/pack cycling (charging/discharging) data (specific capacity and voltage profile) can serve as estimations of active lithium loss and basic battery integrity assessment. Besides, microscope characterization (e.g., scanning electron microscope) and composition analysis can be further applied to ascertain the morphology degradation and Li/TM dissolution. Though, it should be noted that the latter two diagnosis methods are commonly used in academic studies, but might not be suitable for industrial practice due to high cost and equipment unavailability.

Direct regeneration methods can be categorized into two routes: regeneration and upcycling. The former aims to repair the chemical composition, impurity phase, and particle morphology of cathode materials and fully recover the electrochemical performance to the original level; the latter aims to upgrade existing cathode materials into materials that exhibit superior electrochemical properties or applicability. These direct regeneration methods will be discussed in detail in the following sections.

3.1 Cathode regeneration

Cathode direct regeneration methods are based on repairing spent active cathode materials without destroying their original crystal structure and components. Thus, cathode regeneration techniques require an in-depth understanding of the failure or degradation mechanisms of cathode materials after thousands of charge–discharge cycles. The possible failure/degradation mechanisms of the cathode materials of EOL LIBs are summarized in the previous section; the dominant detrimental phenomena observed in spent cathode materials are Li-loss, impurity accumulation, and particle cracking, causing low reversible capacity and poor cycling stability. It has been reported that solid-state sintering, hydrothermal treatment, chemical/electrochemical lithiation, the molten salt sintering method, or combined processes can effectively repair the composition and morphology degradation, remove inactive impurities, and recover the electrochemical performance of spent cathode materials (details listed in Table 1).

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Solid-state sintering can be used to revive degraded cathode materials via a high-temperature sintering procedure with the addition of the required constituent elements (Li and/or TMs). It should be noted that the sintering process during regeneration is similar to that used for cathode synthesis, in which case Li salts and hydroxide precursors are sintered to obtain layered oxide materials or olivine cathode materials. Thus, solid-state sintering with the addition of lithium sources (LiOH and Li₂CO₃) can be used to heal the compositions of spent cathode materials easily. Furthermore, the high-temperature treatment can also provide a suitable condition for recrystallization to eliminate possible intracrystalline defects and repair degraded morphology. Sun et al. reported that spent NCM cathode materials, harvested from commercial pouch cells, could be successfully regenerated via a mechanochemical activation-assisted solid-state sintering process (Fig. 6(A)).⁴⁶ After high-temperature treatment with the addition of Li₂CO₃, the specific capacity and cycling performance of the regenerated NCM materials were evidently recovered to that of the pristine cathode materials. By sintering with manganese acetate (Mn(CH₃COO)₂) and LiOH, the Mn-loss and Li-loss defects in spent LMO cathode materials and particle cracks can be recovered at the same time.³⁷ Incorporating intensive mechanochemical activation (ball-milling) prior to solid-state sintering procedures has also been confirmed to be able to effectively improve lithium-ion diffusion with smaller particle size.^46,49 Beyond these reports, considering that the Li⁺ transport can be impeded by the strong electrostatic repulsion from a TM octahedron in the Li layer provided by the inactive impurity phases (rock-salt/spinel), Zhou and coworkers proposed a novel topotactic solid-state sintering regeneration method for spent layered oxide materials.³⁴ Li-poor phases (rock-salt/spinel), formed on the surface of the cycled cathode materials, were firstly treated with ammonium hydroxide to get TM hydroxides (TM(OH)₂) ready for further lithium replenishment, and then back to NCM/LCO cathodes via a facile solid-state sintering process with the addition of LiOH. Solid-state sintering is applicable to various cathode materials with different structures (Table 1). Considering that the essential recrystallization processes of spent cathode materials need high-temperature environments, other direct regeneration methods usually need to be combined with additional high-temperature annealing processes.

For hydrothermal regeneration, the spent cathode materials are immersed into specific solutions containing a high concentration of lithium sources and then the suspensions are sealed in heated autoclaves for relithiation reactions (Fig. 6(B)). It was first reported in 2004 that a spent LCO cathode could be renovated by immersing the degraded LCO in a 5.0 M LiOH solution and heating at 200 °C for 20 h. Multiple studies have been published since then on the optimal choice of lithium salts,⁵⁰ solvents,⁵¹ additional reductants,⁵² heating temperatures,⁴⁰ and use of auxiliary means (ultrasound^53,54) for hydrothermal treatment processing of spent cathode materials. It should be noted that the Li-deficient phases (Li_yTMO₂ or Li_yFePO₄, y < 1), where the original structures of layered TM oxides or olivine phases are still intact but with lithium vacancies, can be effectively healed via long-duration hydrothermal lithium replenishing treatments with the aid of ultra-high solubility of lithium salts in supercritical water. Nevertheless, for Li-poor phases (including lithiated spinel (Li_xTM_3−xO₄) and rock-salt (Li_xTM_1−xO)) and irreversible impurity phases (such as TM₃O₄, and TMO) in spent cathode materials, where lithium vacancies are occupied by TMs and the original structures are transformed into spinel/rock-salt phases, additional annealing procedures are needed to provide high-temperature environments for the corresponding relithiation and recrystallization processes.^43,55 Besides, considering the high price of lithium, the concentrated solution of lithium salts used in hydrothermal treatment is also a nonnegligible challenge. Moreover, organic solvents used in ionothermal lithiation treatments are usually toxic and expensive and the following disposal of such solvent wastes is also an extra burden on the environment.

Lithiation methods, including electrochemical and chemical lithiation, can compensate for the loss of active lithium in degraded cathode materials. Prelithiated graphite anodes and separators have been reported to be able to regenerate degraded LFP cathode materials, where the active materials did not need to be peeled or dissolved off from the current collectors (Fig. 6(C)).⁴⁴ Supplementary lithium ions from prelithiated graphite anodes or separators can be intercalated into cycled cathode materials with Li vacancies during discharging. It has also been reported that mediators based on chemical redox reactions are necessary for lithium ions and electrons from a lithium source to the cathode (Fig. 6(D)). A post-sintering treatment is then required to remove the cracks in secondary particles.⁴⁷ Unfortunately, individual chemical/electrochemical lithiation regeneration methods are unable to recover inactive decay phases and degraded morphology. Besides, considering the complicated prelithiation of the anode/separator and unstable lithiation reagents, lithiation regeneration methods are likely to be impractical in industry at present.

In molten salt regeneration methods, spent cathode materials are mixed with an excess amount of well-designed eutectic Li salt mixtures (e.g., LiNO₃/LiOH⁴⁸ and LiOH/KOH⁵⁶), and then the spent cathode materials are relithiated in the resulting mixture by heating at a low temperature for a long duration and washed with deionized water to remove residual Li salts. After a short period of high-temperature annealing, the molten salt-regenerated cathode materials can successfully regain their original composition and crystal structures, leading to effective recovery of their specific capacity, cycling stability, and rate capability to the levels of the original materials (Fig. 6(E)).⁴⁸ The selected eutectic molten salts usually have low melting points, which provide a suitable environment and reaction interfaces for ion migration into the degraded cathode materials at low temperatures. For this purpose, excessive amounts of Li salts must be used in the molten salt sintering regeneration process and most of the salts are washed away from the regenerated cathode materials, causing either severe waste of lithium salt or additional lithium salt recycling processes.

Restoration to the targeted composition and recrystallization are two necessary processes to recover spent cathode materials with acceptable electrochemical performance. In this case, solid-state sintering technology is superior to hydrothermal, chemical/electrochemical lithiation and eutectic molten salt sintering routes, whereas high-temperature annealing can both enable replenishment lithiation and accelerate crack repairing and recrystallization to remove inactive/Li-poor impurities.

3.2 Cathode upcycling

Degraded cathode materials can be directly repaired via the above-mentioned regeneration technologies and their electrochemical performance can be revived to their original commercial levels. During these direct regeneration processes, Li vacancies, inactive impurity phases, and degraded particle morphology can be repaired and recovered via a lithiation reaction and recrystallization under high-temperature treatment. Furthermore, degraded cathode active materials can be upcycled to advanced materials with superior electrochemical performance via tiny modifications to the existing regeneration methods (Fig. 7(A)).⁶⁰ Spent cathode materials can thus be repaired and upgraded in a one-step process. The regenerated cathode chemistry could provide highly ranked materials 10 years ago, but may be lagging behind for the current application standards. Thus, an upgrading of the outdated cathode chemistry is needed to give it a new life. Considering the development of commercial LIB products, which are characterized either by cathode chemistries or crystallinity (Fig. 7(B and C)), there is no doubt that cathode upcycling has notable advantages in terms of higher added value/interest, excellent adaptability to development and promoted sustainability of the LIB industry.

Cathode upcycling can be realized via multiple technical routes, including doping, coating, particle size adjustment, and composition modification (details listed in Table 2). It has been reported that a degraded LCO cathode with poor tolerance to high-voltage cycling can be repaired and upgraded to a co-doped LCO via sintering with spent NCM cathode derivatives and Li₂CO₃ at high temperatures (Fig. 7(D)).⁵⁷ The upcycled high-voltage LCO with Ni/Mn substitutional dopants in the Co layer enhances Co–O bonding, which suppresses oxygen release and harmful phase transformation during delithiation, thus stabilizing the layered structure and leading to superior electrochemical performance at 4.6 V. Besides, metal element (such as Mg,^35,61,62 Ti,⁶² Na,⁶³ Al,³⁵ and V⁶⁴) and polyanions (PO₄³⁻ anions⁶⁵) dopants have also been proven to be able to boost the electrochemical performance of regenerated cathode materials via improvement of ion-diffusion and structural stability. However, adding one or several kinds of dopants to cathode materials may increase difficulties in the subsequent recycling or regeneration processes if the add-on substances are no longer needed. Based on this concern, Zhou and coworkers proposed a dual-doped upcycled LCO cathode with ultra-stable cycling performance at high voltages via introducing only a tiny dose of dopants (as low as 0.9 wt%) to the original cathode constitution.³⁵

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A functional coating layer is another effective way to upgrade spent cathode materials. For a degraded LFP cathode, compensation of Li loss and an N-doped carbon coating layer can be realized via a combined hydrothermal and high-temperature sintering process (Fig. 7(E)).³⁶ The upcycled LFP cathode exhibits enhanced cycling stability compared to the original LFP cathode, where the Li⁺ diffusion kinetics are improved, leading to excellent reversibility of the phase transition. A regenerated LCO cathode with an over-lithiated layered oxide (Li_1.20Mn_0.54Co_0.13Ni_0.13O₂) coating layer was also reported to have excellent voltage stability and high energy density with a protective high-voltage-stable-shell and a well-ordered layer-LiCoO₂ as the high-capacity-core.⁶⁶ Surface coating is highly effective to stabilise the surface of cathode particles, enhancing the storage and electrochemical performance,^67,68 and the coating candidates are various, including oxides, phosphates, fluorides, lithium-containing composite oxides, and conductive polymers.⁶⁹ Usually, the coating processes (such as hydrothermal and high-temperature annealing) can be readily incorporated with the existing regeneration methods, which is obviously beneficial to the industrial practices to obtain high-value-added upcycled products without additional procedures. Thus, the functional layer coating technique shows promising potential for spent cathode upcycling. Yet, the coatings are mostly hard to remove without destroying the cathode particles and are not effective dopants, so it may bring difficulties in the subsequent recycling. Sustainable close-loop upcycling routes based on doping/coating techniques should be further verified and the influence of dopants and coatings on the whole regeneration/upcycling cycles also needs careful evaluation.

Particle size adjustment and compositional upcycling methods have been reported to have decent upcycling effects on cycled cathode materials with layered oxide strutures.^58,59,70Via a facile and effective strategy, sintering with low-melting-temperature eutectic lithium molten salts, the spent polycrystalline ternary cathode materials were transformed to plate-like single-crystal materials with excellent rate performance and outstanding cycling stability (Fig. 7(F)).⁵⁰ Molten-salt-based sintering methods can also be employed to alter the chemical compositions of the regenerated cathode materials toward higher energy density. Dai and coworkers developed a reciprocal ternary molten salts system (i.e., Ni(NO₃)₂, LiCl, and NaOH) to directly upcycle spent Ni-poor NCM111 to Ni-rich NCMs with element homogeneity by simultaneously realizing the addition of Ni and the relithiation of Li in spent NCM111 cathode materials (Fig. 7(G)).⁵¹ During these reported processes, replenishment of Li-loss and healing of particle cracks can be easily achieved by using excess Li-containing molten salts and high-temperature environments. In addition, heat treatments with deliberately designed combinations of multiple molten salts have been confirmed to be able to convert spent polycrystalline Ni-lean cathodes into single-crystal Ni-rich cathodes in a one-step molten salt sintering method.^71,72 No additional element was introduced in the molten-salt-based sintering methods, which is favorable to cost-reduction and simple upcycling. However, the obviously overdosed Li and Ni salts, aiming to accelerate ion diffusion, will cause extra waste or additional recycling, definitely increasing the cost and compromising the sustainability. The kinetics mechanism of Ni diffusion into a Ni-poor layered oxide with the aid of molten salts and the following recrystallization processes of high-Ni LiTMO₂ are worth probing into. Moreover, for olivine cathodes, upcycling spent LFP to high-energy-density LFMP cathode materials should be technologically viable, but yet have not been reported so far. Future research focusing on lowering the usage of or recycling the molten salts, ion diffusion kinetics inside molten salts, and upcycling of olivine cathodes is encouraged.

4. Perspectives

Efficient direct cathode regeneration and upcycling technologies can at least alleviate or potentially solve the pressing concerns caused by extensive applications of LIBs, such as erratic supply of raw materials and disposal of EOL batteries. Considering the frequent technological progress and rapid product iteration of LIBs, the cathode upcycling method is showing excellent flexibility to upgrade spent cathodes with superior electrochemical performance and boost the LIB industry towards a circular economy instead of the resource-based economy. Besides, solid-state sintering regeneration methods are showing advantages in targeted chemistry restoration and recrystallization and more importantly, they can be easily incorporated with the existing manufacturing processes and equipment.

Progress and proof have been made to justify the technical feasibility of cathode repairing/upcycling methods towards recovered/upgraded electrochemical performance of spent cathodes, adaptability for various types of active materials, and commendable added value for the regenerated/upcycled materials. Still, it should be recognized that currently available cathode regeneration/upcycling technologies are still low in technology readiness level. More down selection and maturation effort is needed for further improvement of scalability, profitability, and quality control. In particular, highly demanding complicated pretreatment processes are prominent issues for the following promotion of direct recycling technology, as they determine the quality of the regenerated materials, and associated difficulties in regeneration/upcycling procedures. Innovation and optimization of non-destructive efficient component separation and detrimental impurity control will definitely increase the industrial scalability and realize the anticipated high profit of direct recycling.

The scientific challenge for recycling lies in a low cost, effective processing for achieving efficient mass transport across a heterogeneous solid/solid interface. The detailed mass transport includes the following possibilities.

(1) Lithium replenishment is ubiquitous in cathode regeneration of various active materials, which is usually driven by lithium concentration difference. Besides simple restoration of stoichiometric ratio (Li:TMs), a high-temperature environment for layered structure recrystallization is also indispensable to further raise layered ordering and reduce defects (e.g., cation mixing and crack morphology).

(2) Reordering TMs inside an oxide framework. Spinel and rock-salt structures are generally seen in spent cathodes. Both direct regeneration and upcycling require pushing the dislocated TMs back to their TM layers – to form the targeted layered structures.

(3) Add-on substances, either dopants or coating precursors, are promising solutions for cathode upcycling. Controlling the distribution of such add-on substances needs to be achieved by developing the corresponding processes. For instance, we will need to develop compatible low-temperature and rapid processes to reduce the cation inter diffusion for coating-based strategies. Alternatively, we need to encourage inter-diffusion for doping strategies.

(4) Impurity control is a common scientific challenge in pretreatment procedures for all recycling processes. Here, we have to admit that not all impurities are detrimental. We can hypothetically inherit some characteristic impurities from recycling processes. Impurities can include Cu, Al, carbon, metal fluorides and metal oxides from CEI, potential decomposition products from LiPF₆, and so on. The real challenges are (a) to profile all these impurities; (b) to separate the beneficial ones from detrimental ones; and (c) to mitigate the detrimental ones. Making it even more of a challenge, the electrochemical functionality of impurities depends on their locations: dopants in the lattice, residuals in the grain-boundary, or the surface impurities.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We would like to show gratitude to the National Natural Science Foundation of China (no. U21A20170 (X. He), 52206263 (Y. Song), 22279071 (Y. Wu) and 22279070 (L. Wang)), and the Ministry of Science and Technology of China (2019YFA0705703 (L. Wang)). The authors thank Joint Work Plan for Research Projects under the Clean Vehicles Consortium at U.S. and China-Clean Energy Research Center (CERC-CVC2.0, 2016–2020). We also would like to thank the “Explorer 100” cluster system of Tsinghua National Laboratory for Information Science and Technology for facility support. Research at Argonne National Laboratory was funded by the US Department of Energy, EERE Vehicle Technologies Program. Argonne National Laboratory is operated for the US Department of Energy by UChicago Argonne, LLC, under contract DE-AC02-06CH11357.

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