Direct recycling of degraded Ni-rich cathodes: recent advances in regeneration and upcycling

Gui Chu ^a, Yu Huang ^a, William Hawker *^d, Lianzhou Wang *^bc and Xiaobo Zhu *^a
aCollege of Materials Science and Engineering, Changsha University of Science and Technology, Changsha, 410114, P. R. China. E-mail: xbzhu@csust.edu.cn
^bNanomaterials Centre, School of Chemical Engineering, Australian Institute of Bioengineering and Nanotechnology, The University of Queensland, Brisbane, QLD 4072, Australia. E-mail: l.wang@uq.edu.au
^cDept of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, China
^dPure Battery Technologies, Brisbane, QLD 4000, Australia. E-mail: will@purebatterytech.com

First published on 31st July 2025

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Gui Chu

Gui Chu is a master's student in Materials and Chemical Engineering at Changsha University of Science and Technology. Under the supervision of Professor Xiaobo Zhu, his research centers on the experimental and theoretical investigation of cathode materials for lithium-ion batteries.

Yu Huang

Yu Huang is a master's student in Materials Science and Engineering at Changsha University of Science and Technology, supervised by Professor Xiaobo Zhu. Her research is dedicated to the synthesis and optimization of layered cathode materials for alkali-ion batteries.

William Hawker

William Hawker is the Chief Technology Officer at Pure Battery Technologies, a process technology company focused on commercialising efficient and effective processes of refining primary and recycled materials directly to battery material products. His research interests include thermodynamic modelling of complex aqueous solutions, combining process system simulation with economic evaluation for system optimisation, and development of innovative extraction, purification and refining processes for critical minerals and metals.

Lianzhou Wang

Lianzhou Wang is a chair professor at the Dept. of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University and a honorary professor of The University of Queensland (UQ), Australia. His research interests include the design and application of semiconductor nanomaterials for solar energy conversion/storage systems including photocatalysis, photoelectrochemical devices and rechargeable batteries.

Xiaobo Zhu

Xiaobo Zhu attained his PhD degree from The University of Queensland in 2018. He is a full professor at Changsha University of Science and Technology. His research focuses on synthesis, characterization, and application of inorganic materials for electrochemical energy storage.

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1. Introduction

Lithium-ion batteries (LIBs) are crucial components of the modern energy landscape, powering a broad spectrum of applications from portable electronics to electric vehicles (EVs) and grid-scale energy storage systems. Driven by the global transition toward zero-emission transportation and renewable energy integration, the demand for LIBs is projected to exceed 2.5 TWh by 2030, representing more than a fivefold increase from 2022 levels (Fig. 1a).¹ This dramatic growth underscores the critical need to establish sustainable battery production and end-of-life management to mitigate resource depletion and environmental burdens.² According to the International Energy Agency (IEA) forecast (Fig. 1b), the amount of spent LIBs from EVs and storage is projected to witness a 80-fold growth from 2020 to 2030, and nearly a 1000-fold growth by 2040, within its Sustainable Development Scenario.³ This exponential increase in end-of-life batteries presents both a significant environmental challenge and a critical opportunity for resource recovery.

Central to LIBs is the cathode active material (CAM), which governs battery performance, cost, and environmental impact.^4,5 CAMs are typically lithiated transition metal (TM) compounds categorized by their crystal structure into major types, including layered oxides (e.g., LiCoO₂ (LCO), LiNi_xCo_yMn_1−x−yO₂ (NCM), LiNi_xCo_yAl_1−x−yO₂ (NCA), Li-rich Mn-based layered oxides), spinel oxides (e.g., LiMn₂O₄ (LMO), LiNi_0.5Mn_1.5O₄ (LNMO)), and olivine-type phosphates (e.g., LiFePO₄ (LFP), LiFe_xMn_1−xPO₄ (LFMP)).^6,7 Currently, the commercial CAMs are dominated by NCM/NCA and LFP due to their intensive use in EV batteries. While LFP is notable for its cost efficiency and electrochemical/thermal stability, NCM/NCA offers superior energy densities.^8,9 For example, as the Ni content increases, the energy density rises from 564.1 Wh kg⁻¹ for NCM111 (LiNi_1/3Co_1/3Mn_1/3O₂) to 619.4 Wh kg⁻¹ for NCM523 (LiNi_0.5Co_0.2Mn_0.3O₂) and 764.7 Wh kg⁻¹ for NCM811 (LiNi_0.8Co_0.1Mn_0.1O₂), compared to 517.1 Wh kg⁻¹ for LFP (Fig. 1c).¹⁰ NCM/NCA containing ≥50% nickel in transition metals (TMs) are generally classified as Ni-rich layered oxides (NRLOs).¹¹ This advantage in energy density makes NRLOs indispensable for weight- and volume-sensitive applications, particularly long-range EVs.^12,13 As illustrated in Fig. 1a, NRLOs are projected to continue dominating battery demand in the foreseeable future.

However, the widespread adoption of NRLOs is hindered by both economic and technical challenges. High production costs stem from the elevated prices of nickel (∼18 USD kg⁻¹), comparable to cobalt (∼24 USD kg⁻¹), as well as the reliance on lithium hydroxide (LiOH), a more expensive Li source than lithium carbonate (Li₂CO₃) used for the production of lower-Ni cathodes.¹⁴ Additionally, the synthesis of NRLOs often necessitates oxygen-rich atmosphere rather than ambient air to maintain high-valence Ni states and suppress Ni²⁺/Li⁺ cation mixing, further driving costs upward.^11,15 Fig. 1d presents the current prices of three representative NRLOs,¹⁶ along with the breakdown of raw material cost contributions. It is evident that as the Ni content increases from NCM523 to NCM811, a significant portion of the overall cost rise stems not only from raw materials but also from increased processing requirements. Their high reactivity with ambient moisture further necessitates stringent environmental controls during storage and handling, adding to operational costs.^17–19 Performance degradation during storage and battery cycling is another limitation of NRLOs. These materials undergo a series of irreversible transformations including phase transitions (e.g., layered-to-spinel or rock-salt), particle fracturing, TM dissolution, lithium/oxygen loss, etc. Collectively, these degradation pathways accelerate capacity fade compared to LFP or lower-Ni CAMs, leading to premature battery retirement and amplifying both commercial and environmental concerns.

Currently, the dominant recycling approaches for spent LIBs—pyrometallurgy and hydrometallurgy—are designed primarily for elemental recovery.^22–25 As schematically shown in Fig. 1e, pyrometallurgical processes involve high-temperature smelting that recovers Ni, Co, Cu, Li, Al as alloys or in slag, requiring further extraction and purification. Hydrometallurgical methods, in contrast, use leaching and solvent extraction to isolate target elements but generate significant secondary waste and reduce high-value CAMs to metal salt precursors. Although both are widely implemented, they fail to preserve the structure or electrochemical function of cathode materials, diminishing their value return.

In response to these limitations, direct regeneration—and more recently, direct upcycling—has emerged as a promising closed-loop recycling strategy aimed at restoring or even enhancing the structure and performance of spent cathode materials.^26–34 As shown in the inset of Fig. 1e, a surge in related publications retrieved from the Web of Science database highlights growing research interest in the direct regeneration and upcycling of CAMs. Unlike conventional recycling routes that decompose materials into elemental constituents, direct regeneration and direct upcycling address specific degradation mechanisms—such as lithium loss, phase transitions, or surface contamination—through targeted post-treatment. Direct regeneration focuses on restoring the spent cathode to its original composition and structure, while direct upcycling intentionally modifies or upgrades its chemical or structural features to achieve improved performance. This capability is particularly significant for NRLOs, as upcycling offers a breakthrough pathway to mitigate their inherent challenges, such as high production costs and susceptibility to severe degradation, by transforming spent materials into higher-value, more robust products that can even surpass original performance baselines.^35–37 These approaches enable the reuse of high-value cathode materials with significantly reduced environmental impact, energy consumption, and production cost. Taking NCM523 as an example, direct regeneration show clear advantages in terms of less greenhouse gas (CO₂) emissions and energy consumption (Fig. 1f and g).²⁰ As shown in Fig. 1h, techno-economic analysis also demonstrates the financial benefit of direct recycling over pyrometallurgical and hydrometallurgical methods, with direct recycling of NRLOs including NCM622, NCM811, and NCA proving much more profitable than for LFP and LMO.²¹ Given the rapid deployment of NRLOs and their considerable manufacturing expense, direct regeneration and upcycling represent especially compelling pathways for sustainable and economically viable cathode material recovery.

This review presents a comprehensive overview of recent advances in the direct regeneration and upcycling of NRLOs cathode materials. We begin by examining the key degradation mechanisms affecting NRLOs during storage and cycling—specifically phase evolution, stoichiometric imbalance, surface contaminations, and morphological damage. Unlike previous reviews detailing the degradation mechanisms,^{8,9,11,12,38,39} this work contextualizes these structural and chemical changes specifically in relation to the design and effectiveness of recovery strategies. Subsequently, we evaluate contemporary direct regeneration techniques—including solid-state sintering, hydro/solvothermal treatment, molten-salt fluxing, chemical lithiation, and electrochemical lithiation—assessing their efficacy in restoring both the structural integrity and electrochemical performance of spent NRLOs. Furthermore, we explore recent advances in direct upcycling, an approach that moves beyond simple restoration to actively enhance the properties of degraded CAMs. This strategy offers novel pathways to potentially mitigate intrinsic NRLO limitations by strategically leveraging the structural or chemical features present in the spent material itself, employing techniques such as compositional tuning, morphological reconstruction (e.g., single-crystallization), targeted elemental doping, and functional surface modifications. Finally, we identify the key challenges and outline future opportunities for advancing direct regeneration and upcycling technologies, aligning these developments with the growing imperative for sustainable LIB recycling and the establishment of closed-loop material flows within the battery industry.

2. Chemical/structural degradation of NRLOs

NRLOs, despite offering high specific capacity and energy density, are intrinsically susceptible to multiple degradation pathways during both electrochemical cycling and ambient storage. Their inherent chemical and structural instability, particularly pronounced at elevated Ni content, accelerates material deterioration and performance decline. These degradation phenomena can be broadly categorized into four interrelated aspects: (1) phase evolution, (2) stoichiometric imbalance, (3) surface contamination, and (4) morphological damage. A clear understanding of the resulting physical and chemical characteristics of degraded NRLOs—shaped by these interconnected phenomena—is crucial not only for engineering effective direct regeneration processes aimed at restoring the original structure and function but also for developing advanced direct upcycling approaches designed to enhance material properties beyond their initial baseline.

2.1 Phase evolution

Phase evolution in NRLOs predominantly arises from lattice instability under deep delithiation. During charging, Li⁺ extraction from NRLO is accompanied by a series of phase transitions from the original hexagonal phase (H1) to a monoclinic phase (M), and subsequently two rearranged hexagonal phases (H2 and H3) at progressively high potentials.⁴⁰ Initially, the c-lattice parameter gradually increases with Li⁺ extraction due to increased repulsive interactions between negatively charged TMO₂ slabs. However, continued Li⁺ extraction through the H2 → H3 phase transition results in a dramatic decrease in the c-axis. Fig. 2a illustrates the lattice variation of NCM811 alongside its dQ/dV profile, the c-lattice increases from 14.23 Å to 14.47 Å (4.01 V), followed by a slight decrease until 4.2 V. Then it experiences a rapid drop to 13.96 Å at 4.4 V.⁴¹ This contraction is attributed to the severe shrinkage of the Ni ion octahedral cell upon oxidation (losing e_g electrons) and the concurrent oxidation of intermixed Ni²⁺ in the Li layer.⁴²

From a chemical perspective, Ni⁴⁺ in H3 or even the O1 phase of fully delithiated TMO₂ is unstable that increases the covalent character of Ni–O bonds. This facilitates electron transfer from lattice oxygen (O²⁻) to Ni⁴⁺, leading to the oxygen release from the lattice and the reduction of Ni⁴⁺ back to the more stable Ni²⁺ state.^43,44 Owing to their similar ionic radii (r(Ni²⁺) ≈ r(Li⁺)), these newly formed Ni²⁺ ions tend to migrate into vacant lithium sites within the layered structure. This cation mixing, coupled with lattice densification, transforms the ordered layered arrangement into electrochemically inactive spinel or rock-salt phases (Fig. 2b).⁴⁰ Such phase evolution is particularly prominent near the particle surface, exacerbated by high states of charge during cycling.^45–47 Additionally, analogous phase evolution can occur during ambient storage via surface reduction of Ni³⁺, generating NiO-like rock-salt phase independent of electrochemical processes.⁴⁸

These newly formed phases are electrochemically inert and lack continuous pathways for Li⁺ diffusion, which degrades battery performance (e.g., capacity loss, increased impedance) and physically hinders lithium reinsertion during subsequent regeneration attempts. Consequently, effective direct regeneration strategies must aim to reverse these structural transformations and recover electrochemical activity by reconstructing the desired layered framework. Beyond mere restoration, direct upcycling approaches may integrate structural stabilization mechanisms—such as engineering the grain boundaries, inducing single-crystal morphology, or incorporating specific dopants—to suppress subsequent phase transitions during operation, thereby offering the potential to exceed the original performance baseline and extend the cycle life of the recycled material.

2.2 Stoichiometric imbalance

Stoichiometric imbalance in NRLOs refers to deviations from the ideal Li:TM:O ratios, originating from both electrochemical cycling and ambient degradation. The irreversible phase transitions discussed above inherently involve both oxygen loss and lithium depletion, as the resulting spinel or rock-salt phases are lithium-deficient and no longer capable of Li⁺ reinsertion. Simultaneously, parasitic reactions occurring at the electrode–electrolyte interfaces also contribute to stoichiometric shifts.^49,50 Within LIBs, the hydrolysis and oxidative decomposition of electrolyte generate corrosive hydrofluoric acid. HF readily attacks the NRLO lattice, leading to the leaching of TMs into the electrolyte.⁵¹ These dissolved TM ions can migrate through the separator and deposit onto the anode surface (e.g., as metallic clusters), where they can catalyze further electrolyte decomposition and promote hazardous Li metal plating, thereby accelerating the consumption of active lithium from the cathode inventory.⁵²

For severely degraded NRLOs, successful regeneration therefore necessitates not only the replenishment of lost lithium but often also the precise assessment and rebalancing of the TM composition, typically achieved by supplementing deficient elements (e.g., adding Ni sources during relithiation) in appropriate ratios. Direct upcycling leverages this need for compositional control not just for restoration but as an opportunity for targeted performance enhancement. For instance, strategies can involve intentionally increasing the Ni content, introducing specific stabilizing dopants during the regeneration process, or potentially even harnessing beneficial impurity elements already present within the reclaimed “black mass”. These approaches aim to boost specific capacity, improve thermal/structural stability, or impart other desirable properties, thereby shifting the objective from simple repair towards value-added material engineering.

2.3 Surface contamination

Surface contamination on degraded NRLOs stems from two main sources: (i) the formation of residual lithium compounds (RLCs) during exposure to ambient air, and (ii) the accumulation of interfacial by-products generated from interfacial reactions between the cathode surface and the electrolyte in LIBs. RLCs—primarily LiOH and Li₂CO₃—arise not only from direct reactions between excess surface lithium (often intentionally introduced during synthesis to offset Li volatilization at high temperatures) and atmospheric H₂O/CO₂, but also through spontaneous H⁺/Li⁺ exchange between adsorbed moisture and the NRLO surface lattice.¹¹ When assembled into LIBs, these compounds can react detrimentally with electrolyte components (especially LiPF₆), stimulating interfacial reactions.

Once NRLOs are assembled into an LIB, additional surface contamination occurs via the dynamic formation and evolution of the cathode–electrolyte interphase (CEI). The CEI is a complex, heterogeneous layer composed of various organic species (e.g., lithium alkyl carbonates, lithium alkoxides, polymers) and inorganic species (e.g., LiF, Li₂CO₃, TM fluorides, and Li_xPF_yO_z) resulting from electrolyte oxidation and decomposition reactions at the cathode surface.⁵³ While a thin, stable, and ionically conductive CEI can initially passivate the surface from further electrolyte decomposition, excessive or uncontrolled buildup leads to increased interfacial impedance, hindered Li⁺ transport kinetics, and continuous consumption of active materials (both Li and TM).

During direct recycling workflows, surface contaminants—particularly persistent fluorine- and phosphorus-containing species derived from the CEI—often remain adsorbed on or embedded within the reclaimed black mass. These residues can be trapped within particle cracks, along grain boundaries, or inside internal voids, making their complete removal by simple washing procedures challenging. As a result, they can inadvertently become uncontrolled elemental impurities in the final regenerated NRLOs. From an upcycling perspective, however, there is emerging interest in whether these deeply integrated species could potentially be leveraged for functional benefit. For example, elements like fluorine, if accurately quantified and strategically retained or incorporated in a controlled manner, might theoretically enhance surface passivation or improve thermal stability. However, realizing such benefits demands precise compositional analysis, rigorous process control, and a thorough understanding of the distribution and chemical states of impurities. Without careful management, these same impurities are far more likely to induce undesirable side reactions and increase interfacial resistance, compromising the electrochemical performance of the regenerated material. Therefore, upcycling strategies aiming to harness embedded impurities must be grounded in robust characterization, well-calibrated processing, and potentially adaptive design frameworks to ensure the reliability and consistency of the final NRLO products.

2.4 Morphological damage

Morphological degradation, manifesting predominantly as particle cracking, represents one of the most significant and persistent challenges limiting the practical application and long-term cycling stability of NRLO cathodes.^54–57 A key driver of this mechanical instability is the pronounced anisotropic lattice contraction associated with the H2 → H3 phase transition, which occurs at high states of charge (deep delithiation). This abrupt structural transformation generates substantial internal stress within the NRLO particles. The accumulation of this stress over repeated charge–discharge cycles eventually leads to the initiation and propagation of microcracks. The problem is particularly severe in materials with nickel content exceeding ∼80%, such as NCM811 and other high-Ni analogs, where the increased electrochemical activity and associated volume changes exacerbate internal strain.⁵⁸

The development of cracks can lead to particle fragmentation. The loss of electrical contact between fragments and the conductive matrix/current collector results in capacity decay. Furthermore, the formation of cracks facilitates electrolyte infiltration deep into the particle interior. This accelerates parasitic side reactions along the newly exposed surfaces, leading to further TM dissolution, gas generation, and the formation of thick, resistive CEI layers that impede Li⁺ transport along the crack surfaces. Collectively, these chemo-mechanical degradation effects drive rapid structural and interfacial deterioration, ultimately resulting in accelerated capacity fade and premature battery failure. It is noteworthy that even NRLO materials designed as “single-crystals” (intended to eliminate intergranular cracking observed in polycrystalline aggregates) can still suffer from intragranular fractures under demanding conditions,^55,57,59,60 highlighting the difficulty in combating this mechanical degradation of NRLOs.

Therefore, effective direct regeneration strategies must ideally address existing mechanical damage, for instance, through high-temperature annealing aimed at sintering microcracks. However, even successful regeneration processes may leave behind or introduce other structural imperfections—such as tilt boundaries, stacking faults, and residual lattice strain—inherited from the spent material or resulting from the treatment itself. These defects can act as stress concentration points, compromising the long-term mechanical stability of the regenerated material by increasing its susceptibility to fracture during subsequent cycling.^61,62 Consequently, merely repairing apparent cracks may be insufficient. Addressing this persistent challenge often necessitates moving towards upcycling approaches for enhanced mechanical robustness. This requires continued innovation in developing crack-tolerant material architectures (e.g., core–shell structures, concentration gradients), advanced interface engineering techniques (e.g., protective coatings), and precisely controlled synthesis/regeneration protocols designed to minimize detrimental defects. Such advancements are essential to enable the reliable and sustainable reuse of high-performance NRLOs in next-generation LIBs.

As summarized schematically in Fig. 3, the degradation of NRLOs involves a complex interplay of phase instability, stoichiometric deviation, surface reactions, and mechanical failure, presenting a significantly more multifaceted challenge compared to other cathode chemistries like LFP or LiCoO₂ (LCO). Consequently, the rejuvenation of degraded NRLOs requires a holistic approach that addresses not only the replenishment of lost lithium but potentially also the restoration of the layered crystal structure, readjustment of TM stoichiometry, removal or conversion of surface contaminants, and healing of mechanical damage. This complexity underscores the formidable challenges faced in developing robust, efficient, and economically viable direct regeneration and upcycling processes specifically tailored for NRLO materials.

3. Direct regeneration of NRLOs

The pursuit of sustainable LIB lifecycles has catalyzed extensive research into direct regeneration techniques for retired CAMs. These approaches seek to restore the electrochemical functionality of degraded materials without resorting to complete chemical dissolution and resynthesis. Contemporary direct regeneration methods can be systematically classified based on their processing environment and core mechanism into several principal categories: solid-state sintering, hydro/solvothermal treatment, molten-salt fluxing, chemical relithiation, and electrochemical relithiation. These established regeneration methods have been increasingly adapted and investigated for their applicability to the more complex case of NRLOs.

3.1 Solid-state sintering

Solid-state sintering represents a foundational and widely adopted strategy within direct recycling, mimicking the conventional solid-state synthesis routes utilized for producing pristine CAMs. In this method, degraded cathode powder is mixed with a Li precursor and subjected to high-temperature calcination. The general efficacy of solid-state regeneration has been demonstrated for various CAM chemistries, including LCO,⁶³ LFP,⁶⁴ and various Ni-containing layered oxides.⁶⁵ However, applying this method to NRLOs presents distinct difficulties due to their inherent structural instability and multifaceted degradation pathways, as detailed in Section 2. These challenges include the need to replenish the lithium inventory lost during battery operation (relithiation), repair the crystal lattice structure by promoting the reconversion of detrimental secondary phases (e.g., rock-salt, spinel) back to the desired layered structure, and potentially heal morphological defects like microcracks, a process that relies on sufficient mass diffusion under solid-state sintering.

Initial studies established the fundamental feasibility of regenerating NRLOs via solid-state routes. For instance, Zhou et al.⁶⁶ demonstrated that employing a two-step calcination protocol (500 °C for 5 h followed by 900 °C for 12 h) with lithium acetate as the lithium source effectively restored the electrochemical properties of degraded NCM523. Similar outcomes were reported using other lithium precursors like LiOH·H₂O⁶⁷ and Li₂CO₃⁶⁸ for the regeneration NCM523. A primary challenge frequently encountered, however, is the presence of electrochemically inactive impurity phases, particularly rock-salt NiO, formed during degradation. These phases possess poor ionic conductivity and can significantly impede the solid-state diffusion of Li⁺ ions during the annealing process. This kinetic limitation often necessitates extended heating durations or higher calcination temperatures, which can, in turn, lead to undesirable consequences such as particle coarsening, increased levels of Li/Ni cation mixing, and compositional or structural heterogeneity within the regenerated material.

To address these kinetic limitations and enhance the efficiency and homogeneity of solid-state regeneration, various process modifications and pretreatment strategies have been investigated.

3.2 Hydro/solvothermal treatment

Hydrothermal and solvothermal methodologies, widely recognized for synthesizing crystalline nanomaterials with controlled morphology and particle size uniformity, have been adapted for the direct regeneration of spent CAMs. These techniques operate by creating a high-temperature, high-pressure environment within a sealed autoclave, utilizing either an aqueous medium (hydrothermal) or a non-aqueous solvent (solvothermal). Under these conditions, the properties of the solvent, particularly its solvating power and the diffusivity of dissolved species, are boosted. This controlled environment promotes efficient dissolution of lithium precursors, rapid mass diffusion to and within the degraded cathode particles, and facilitates uniform crystal growth processes necessary for structural restoration. Consequently, hydro/solvothermal approaches offer the potential for effective lithium replenishment and structural repair under relatively mild conditions.

The applicability of hydrothermal treatment for NRLO regeneration was demonstrated by Shi and coworkers.⁸⁰ They investigated the regeneration of NCM111 and NCM523 using an aqueous LiOH solution. Their findings indicated that the NCMs necessitated higher hydrothermal processing temperatures (220 °C) to achieve effective relithiation compared to LCO (which required only 180 °C) (Fig. 5a). This difference was ascribed to potentially slower lithiation kinetics in NCM materials, possibly arising from a higher intrinsic degree of Li/Ni cation mixing. Furthermore, for the higher-Ni NCM523 composition, annealing under an oxygen-rich atmosphere was essential to facilitate the complete transformation of residual rock-salt impurity phases back into the desired layered structure.

Seeking to mitigate the high-temperature requirement, Chen and coworkers⁸¹ explored the addition of reducing agents—specifically ethanol, hydrogen peroxide, or ethylene glycol—into the aqueous lithium precursor solution during hydrothermal treatment. This innovative modification enabled the successful relithiation of NCM111 and NCM622 materials at a significantly lower temperature of 100 °C (Fig. 5b). Intriguingly, in another study, H₂O₂ was identified as a necessary oxidizing agent required to re-oxidize Ni²⁺ species back to the desired Ni³⁺ state within the layered structure, thereby repairing structural defects of degraded NCM622.⁸² This apparent contradiction in the reported function of H₂O₂ highlights the complexity of the underlying redox chemistry occurring during hydrothermal regeneration and underscores the critical need for more precise mechanistic investigations to fully comprehend the role of various additives in facilitating NRLO restoration.

Hydrothermal treatment has also been strategically employed as a pretreatment step to modify the surface of degraded materials, thereby facilitating subsequent high-temperature regeneration processes. Zhou's team⁸³ developed a pretreatment protocol using ammonium hydroxide solution. This treatment was designed to chemically transform the inactive rock-salt or spinel phases present on the surface of spent NCM523 particles into a layered Ni_0.5Co_0.2Mn_0.3(OH)₂ intermediate phase (Fig. 5c). This surface conversion was shown to significantly improve the efficiency of Li⁺ transport during the subsequent high-temperature solid-state regeneration step.

Furthermore, a significant challenge within aqueous hydrothermal systems stems from the intrinsic sensitivity of NRLOs towards water as discussed above. Exposure to the aqueous medium can lead to lithium leaching from the structure and surface degradation, counteracting the relithiation effort. In response to these issues related to aqueous processing, Zhou et al.⁸⁴ investigated a solvothermal approach utilizing a saturated ethanol solution of LiNO₃ for the treatment of degraded NCM at 150 °C (Fig. 5d). The use of ethanol, a non-aqueous solvent with a lower boiling point than water, not only generated higher system pressure at a comparatively lower temperature but, more importantly, effectively suppressed the reverse dissolution of lithium ions due to the lower solubility of lithium salts in ethanol compared to water. This resulted in improved overall lithium retention within the regenerated cathode material. Ionic liquids (ILs) represent another category of alternative, proton-free solvents that have been considered for solvothermal-type processes. In pioneering work by Dai and colleagues,⁸⁵ an IL was employed as a fluxing solvent in conjunction with LiBr as the lithium source to regenerate delithiated NCM111 via a process termed “ionothermal” treatment. While demonstrating feasibility for lower-Ni NCM, the application of ionothermal methods specifically for the relithiation of higher Ni content NRLOs has not yet been reported. Furthermore, it should be noted that many ILs are associated with relatively high production costs, which might potentially limit the economic scalability of ionothermal regeneration methods for widespread industrial application.

Presently, the successful and comprehensive regeneration of NRLOs with very high nickel content (e.g., NCM811 and beyond) using hydro/solvothermal methods remains an area with limited reported validation. The increased complexity of degradation mechanisms prevalent in these materials—including more severe phase transformations to rock-salt structures, higher degrees of cation disorder, and extensive formation of microcracks—coupled with their heightened sensitivity towards chemical attack or corrosion induced by protic solvents, likely impacts the full restoration via these techniques. Moreover, a practical constraint is that hydro/solvothermal regeneration protocols frequently necessitate a subsequent solid-state annealing step to achieve complete restoration of crystallinity, remove residual solvent species, and ensure phase purity. While necessary for optimal performance, this additional high-temperature step partially counteracts the primary potential advantages of hydro/solvothermal processing, namely lower energy consumption compared to direct solid-state sintering.

3.3 Molten-salt fluxing

The molten salt (or eutectic medium) method represents a distinct and increasingly investigated approach for direct regeneration. This technique employs mixtures of salts, which exhibit eutectic behavior, meaning their combined melting point is substantially lower than those of the individual salt components. Operating within this liquid-phase ionic environment, which is inherently rich in mobile Li⁺ ions,⁸⁶ offers several potential advantages over solid-state processing. The liquid medium significantly accelerates mass diffusion, facilitates more uniform contact between the lithium source and the degraded cathode particles (including penetration into cracks and pores), and promotes effective particle recrystallization and morphological evolution. Consequently, the molten salt method enables the restoration of lithium stoichiometry and structural integrity under comparatively milder thermal conditions, presenting an energy-efficient and potentially scalable pathway that appears particularly well-suited for the direct regeneration of complex NRLO materials. The specific composition of the molten salt system (binary, ternary, or more complex) can be tailored to govern the melting point, chemical reactivity, and fluxing properties.

3.4 Chemical relithiation

In contrast to the aforementioned regeneration methods that typically rely on elevated temperatures or pressures (solid-state, hydrothermal, molten salt), chemical relithiation presents an alternative strategy predicated on spontaneous, thermodynamically favorable redox reactions occurring under ambient or near-ambient conditions. This approach aims to replenish the lithium inventory within depleted cathode structures without significant energy input, thereby offering a potentially milder and more energy-efficient pathway for regeneration. The core principle involves utilizing chemical reducing agents, often dissolved in appropriate solvents, which can chemically reduce the transition metals in the delithiated cathode material while concurrently facilitating the insertion of lithium ions (sourced either from the reagent itself or from a lithium salt present in the solution).

An early example by Wu et al.¹⁰² utilized specific polycyclic aryllithium compounds (e.g., pyrene–Li, perylene–Li). These organometallic reagents functioned dually as Li sources and reducing agents, reportedly enabling rapid relithiation (∼10 min) of various cathodes including LFP, LCO, and NCM622 under ambient conditions, with the added benefit of recyclable reagents, suggesting potential for sustainable processing. Further exploration has focused on utilizing redox mediators to facilitate electron transfer for lithiation. Expanding this concept, Ko et al.¹⁰³ investigated other organic electron-donating molecules as mediators, including 5,10-dimethylphenazine, ferrocene, and N,N′-diphenyl-p-phenylenediamine. Employed in conjunction with standard Li salts in organic solutions, these mediators were carefully chosen based on their redox potentials to enable controlled, topotactic lithiation of a range of spent NRLOs including NCM622 and NCM811 within their stable voltage window at room temperature, thus offering another potentially scalable route (Fig. 7a). In addition, quinone-based molecules, such as 3,5-di-tert-butyl-o-benzoquinone (DTBQ), were also demonstrated as effective electron shuttles. A pre-formed Li-coordinated DTBQ complex dissolved in an organic solution was used to directly immerse and relithiate Li-deficient NCM622 electrodes (Fig. 7b).¹⁰⁴

While potentially effective for robust structures like LFP where Li deficiency is the primary degradation mode, chemical relithiation alone is often insufficient for highly degraded NRLOs. These materials frequently require a subsequent high-temperature annealing step to fully restore the ordered layered crystal structure and eliminate defect phases formed during cycling. This necessity increases process complexity and partially negates the primary energy-saving advantage of the low-temperature chemical treatment. Furthermore, practical implementation challenges persist regarding the reagents themselves. Many of the effective reagents are costly, reactive organic compounds, raising concerns about handling safety, stability, potential environmental impact from disposal or side reactions, and the efficiency and cost-effectiveness of reagent recycling. These factors currently limit the scalability and industrial viability of chemical relithiation, highlighting the critical need for the development of more benign, cost-effective, stable, and readily recyclable reagent systems, alongside robust and environmentally sound waste management protocols, to fully realize its potential for NRLO regeneration.

3.5 Electrochemical relithiation

Unlike purely chemical approaches driven by reagent redox potentials, electrochemical lithiation is governed by modulating electrical potential or current within an electrochemical cell configuration. Typically, the spent cathode is paired with a counter electrode like metallic lithium foil or a pre-lithiated anode material (e.g., lithiated graphite). By applying an external voltage to drive discharge (lithiation of the cathode) or by harnessing a spontaneous electrochemical potential difference between the electrodes, Li⁺ ions are driven to migrate through the electrolyte and intercalate back into the host structure of the spent cathode, thereby replenishing the lithium vacancies. A key advantage of this approach is the high degree of control it offers; the extent and rate of lithiation can, in principle, be precisely modulated by adjusting electrochemical parameters such as the applied voltage, current density, and the duration of the operation.

Several innovative electrochemical regeneration strategies have been reported. Song et al.¹⁰⁵ developed a room-temperature electrochemical method leveraging spontaneous galvanic corrosion principles to restore Li-deficient NCM622 without external power. In their setup, the aluminum current collector, inherently present with the cathode material, served as a sacrificial anode (reducing agent) due to its electrochemical potential relative to the lithiated cathode (Al/Al³⁺ ≈ 1.37 V vs. Li/Li⁺). This spontaneous electrochemical couple drove the insertion of Li⁺ ions (present in the electrolyte) into the spent NCM622 cathode material (Fig. 7c), recovering electrochemical performance almost equivalent to that of the pristine material. In a distinct approach employing non-conventional electrochemical conditions, Beletskii et al.¹⁰⁶ proposed a plasma electrolysis method for cathode regeneration. This technique involves generating electrical discharges directly within a liquid electrolyte containing the cathode material. The localized plasma zones create extreme conditions characterized by shock waves, high temperatures, and the formation of highly reactive species, including hydrogen atoms and solvated electrons. These synergistic physicochemical and electrochemical effects were reported to facilitate lithium reinsertion while simultaneously removing surface contaminants from composite LiMn₂O₄/NCM622 materials, leading to significant improvements in their power capability and cycling performance (Fig. 7d). Electrochemical methods have also proven useful specifically for surface restoration, particularly for NRLOs degraded by ambient air or moisture exposure. Wang et al.¹⁰⁷ demonstrated that performing several initial galvanostatic charge–discharge cycles within a specific voltage window (3.0–4.5 V) could effectively decompose detrimental Li₂CO₃ residues formed on the cathode surface due to air exposure, thereby restoring the subsequent electrochemical performance in assembled cells.

Overall, electrochemical relithiation operates under mild conditions, potentially saving energy compared to thermal methods, and enables precise Li stoichiometry control without harsh chemical reducers. Its primary limitation, shared with chemical methods, is its focus on Li replenishment; it struggles to remove bulk impurities (NiO, spinel) or repair deep lattice disorder (TM loss, antisite defects). Therefore, post-annealing at high temperatures is frequently required for NRLOs to achieve full structural restoration, adding process steps and energy costs that diminish the low-temperature benefit. Achieving homogeneous lithiation, particularly within large particles or thick electrodes, and carefully managing cell electrochemistry to avoid side reactions remain additional practical challenges for this regeneration approach.

In this section, contemporary direct regeneration methodologies applied to spent NRLOs have been reviewed based on their processing environment. Solid-state sintering represents a widely explored thermal approach, effective for structural repair but often limited by slow solid-state diffusion and heterogeneity. Hydro/solvothermal and molten-salt methods leverage liquid environments to enhance mass transport and facilitate crystal repair under potentially milder conditions, offering versatility but facing challenges related to solvent/salt removal and the common requirement for post-annealing. In contrast, chemical and electrochemical lithiation offer lower-temperature alternatives primarily focused on lithium replenishment, but often struggle with comprehensive structural repair and impurity removal, frequently necessitating subsequent thermal treatment. While significant progress has been made in adapting these techniques for NRLOs by incorporating pre-treatment steps and process modifications, each method presents distinct trade-offs in terms of energy consumption, process complexity, scalability, and effectiveness in addressing the multifaceted degradation of high-nickel materials. A detailed comparison of the reported process parameters and electrochemical performance results across these different regeneration strategies is presented in Table 1.

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4. Upcycling of NRLOs

Distinct from direct regeneration, which aims to restore spent materials to their original state, upcycling targets electrochemical performance beyond the original baseline. This is particularly relevant for NRLOs, offering an opportunity to not only mitigate their intrinsic degradation issues but potentially enhance their properties. Upcycling adapts standard regeneration techniques (e.g., solid-state, molten salt) by incorporating strategic modifications—such as adding supplementary metals or modifying agents (dopants, coatings). Key objectives include tuning composition (e.g., increasing Ni content), improving morphology (e.g., forming single crystals for mechanical robustness), and engineering enhanced structural or interfacial properties. Notably, the degraded state itself, with features like pores and cracks, can be leveraged to facilitate the incorporation of these modifications more effectively than in dense pristine materials; even contaminants might be strategically utilized. Therefore, upcycling presents a compelling pathway to increase the value and sustainability of LIB recycling. The following subsections detail key upcycling strategies applied to NRLOs.

4.1 Compositional upcycling

A primary motivation for compositional upcycling stems from the continuous evolution of NRLO chemistries towards even higher nickel content to achieve greater energy densities. Direct regeneration of current NRLOs like NCM523 yields products potentially misaligned with current market requirements. Compositional upcycling addresses this by intentionally altering the TM stoichiometry during the regeneration process, typically increasing the Ni ratio relative to Co and Mn, to transform outdated feedstocks into contemporary, high-performance compositions (e.g., NCM811, NCM90).

Achieving homogeneous incorporation of the supplementary TMs throughout the existing particle structure is crucial for successful compositional upcycling. Molten salt methods are favored due to the facilitated mass transport in the liquid flux environment, which promotes the diffusion necessary for compositional homogenization alongside structural repair. Numerous studies have demonstrated this approach, successfully converting Ni-lean NCM111 to Ni-rich NCM622 and NCM811.^114–116 The method has also been applied to NRLOs; for example, Qian et al.³⁵ upcycled degraded polycrystalline NMC532 to single-crystal NCM66 and NCM811 by adding specific TM hydroxide precursors to a LiOH–Li₂SO₄ molten system. Similarly, Li et al.¹¹⁷ utilized a LiOH–Li₂CO₃ mixture with added NiCO₃ and MnCO₃ to convert degraded NCM523 into NCM712, achieving performance comparable to commercial counterparts (Fig. 8a). Kim et al.¹¹⁸ implemented a selective lithium extraction pre-treatment via chlorination on degraded NCM622, aiming to homogenize the starting material before proceeding with upcycling to single-crystal NCM811. Recently, Yoon et al.¹¹⁹ introduced a liquified-salts-assisted upcycling approach by using Ni(NO₃)₂·6H₂O as both a Ni source and a eutectic component in combination with a LiOH–LiNO₃ mixture. Aided by planetary centrifugal mixing, the mixture transformed into a liquidized environment for enhanced precursor integration and mass diffusion, enabling efficient conversion of spent NCM523 to single-crystal NCM811.

Overall, compositional upcycling offers a chance to upgrade the formula, especially increasing the nickel fraction for higher capacity. However, achieving the target stoichiometry requires precise control of precursor addition and reaction conditions. Ensuring homogenous composition without inducing detrimental phase separation represents a difficulty, particularly for substantial compositional shifts.¹²⁰ Also, the costs of processing and supplementary TM sources (especially virgin Ni salts) require economic considerations, which may impact the overall viability compared to simpler regeneration or hydrometallurgical routes.

4.2 Morphological upcycling

Independent of, or often concurrent with, compositional changes, upcycling methods frequently target modifications to the crystal shape and particle morphology to enhance performance and durability. A dominant focus is transforming degraded polycrystalline NRLO particles, susceptible to intergranular cracking, into more robust single-crystal structures. Lacking internal grain boundaries, single crystals generally exhibit improved mechanical integrity, enhanced thermal stability, and potentially reduced surface reactivity. This conversion often leverages high-temperature processes, particularly molten salt methods, where the liquid flux environment promotes significant mass transport via dissolution-reprecipitation and Ostwald ripening. Such mechanisms facilitate extensive recrystallization and grain growth, enabling the formation of larger, dense, well-faceted single crystals from polycrystalline or fragmented particles, while inherently aiding the healing of microcracks and voids.

While molten salts readily promote single-crystallization, solid-state annealing can also achieve morphological upcycling but requires careful management of kinetic barriers. Zhou et al.,¹²¹ for example, used segmental calcination for NCM622, suggesting that air passages formed in cracked particles during initial calcination aided lithium transport and structural remodeling from irregular collapsed morphologies to octahedral crystals. Fan et al.³⁶ demonstrated a solid-state approach for severely cracked polycrystalline NCM111, NCM523, and NCM811. They identified the presence of surface rock-salt phases as a primary obstacle hindering crystal growth. By implementing a pre-sintering step to first revive this surface layer, subsequent high-temperature annealing successfully induced the reshaping of the fragmented particles into upcycled single-crystalline NRLOs with substantially improved capacity retention (Fig. 8b).

The control of crystal regrowth and morphological evolution during upcycling remains a significant technical challenge. The initial state and degree of degradation of the NRLO feedstock are often overlooked but inevitably influence the process kinetics and outcomes, adding variability. Moreover, while mechanically beneficial, excessively large single crystals can suffer from slow Li⁺ diffusion kinetics due to long pathways, impairing rate performance and potentially causing heterogeneous lithiation, strain, and fracture.^57,122,123 Therefore, precise optimization of process parameters (temperature, time, atmosphere, salt chemistry etc.) is required. The goal is to achieve not just single-crystallinity, but also optimal particle size, morphology, density, and minimal internal defects.

4.3 Lattice doping during upcycling

Elemental doping is a well-established materials engineering approach used to modify the intrinsic properties of CAMs, including NRLOs, often aiming to enhance ionic or electronic conductivity, improve structural stability against phase transitions, or passivate reactive surfaces. While doping is typically performed during the initial synthesis of pristine materials to ensure homogeneous distribution, incorporating dopants into already formed, well-crystallized structures can be kinetically challenging due to slow solid-state diffusion. However, degraded NRLOs present a unique scenario. The presence of abundant structural defects—such as vacancies, dislocations, microcracks, and internal voids—can potentially serve as facile diffusion pathways, making the integration of dopant elements during the regeneration or upcycling process more feasible. This opens the possibility of simultaneously restoring the basic structure and enhancing its properties through controlled doping.

An intriguing avenue within this technique involves the in situ utilization of impurity elements commonly found in the recovered black mass from dismantled LIBs. These impurities (e.g., Al and Cu from current collectors, Fe from casings, F and P from electrolyte decomposition) are often considered contaminants requiring removal.^124,125 However, if their presence can be managed and their incorporation into the NRLO lattice managed effectively, they could potentially serve as beneficial dopants. Zhang's group³⁷ explored this concept in a two-step regeneration of spent NCM523 using a molten salt reaction followed by thermal treatment. They simulated the incorporation of aluminum impurities as a dopant by Al removal and subsequent addition of Al particles. The electrochemical performance of the Al-doped regenerated NCM compared to regenerated NCM without Al showed improvement (Fig. 9a). Their further mechanistic studies suggested that this in situ Al doping could increase the diffusion barrier for Ni migration, thereby suppressing detrimental Li/Ni cation mixing during cycling of the regenerated NCM523.¹²⁶

Taking this concept further towards a closed-loop system, Zhou's team¹²⁷ intentionally used Al powder and Cu(OH)₂ converted from waste current collector debris as dopant sources during the molten salt upcycling of highly degraded polycrystalline NCM83. Their findings indicated a synergistic effect, with Al³⁺ preferentially occupying TM vacancies (strengthening bonding, reducing antisite defects) and Cu²⁺ occupying Li vacancies (supporting the layered structure, reducing distortion). This dual doping reportedly improved lattice stability, mitigated strain accumulation, stabilized surface oxygen, and enabled stable high-voltage (4.6 V) operation (Fig. 9b).

Beyond cationic doping, the incorporation of specific anions has also been investigated. Species like phosphate or fluoride ions can be intentionally introduced during the process or potentially derived from binder and electrolyte remnants (e.g., LiPF₆ decomposition products). Fan et al.¹²⁸ explored the benefits of phosphate doping during NCM523 regeneration. They suggested that incorporating the large PO₄³⁻ polyanion, with its high electronegativity, could form strong bonds with TM cations (especially Ni), thereby reducing the degree of cation mixing within the layered structure (Fig. 9c). Separately, Zheng et al.¹²⁹ investigated the impact of fluorine impurities, often present in recycled streams, on the repreparation of NCM622 via hydrometallurgical coprecipitation. This study found that low concentrations (up to 1 at%) of fluorine impurity beneficially modified surface chemistry (increasing the Ni²⁺ ratio) and induced particle porosity, enhancing bulk Li⁺ diffusivity and leading to improvements in specific capacity and cycling stability (Fig. 9d).

Integrating elemental doping into the direct regeneration/upcycling workflow presents a promising pathway for enhancing the performance and durability of recovered NRLO materials. While the in situ utilization of beneficial elements from inherent impurities is an intriguing possibility, directly leveraging the variable contaminants present in raw black mass remains challenging due to significant variations in impurity type, concentration, and chemical state. Consequently, most successful strategies to date focus on adding controlled amounts of dopant elements (sourced from virgin materials or purified recycled streams) to relatively clean, pre-treated spent NRLOs. Further research is demanded to optimize controlled additive doping methods and develop advanced strategies for the controlled utilization of specific inherent impurities, addressing feedstock variability to advance upcycling towards practical application.

4.4 Surface modification during upcycling

As detailed in Section 2, degraded surfaces compromised by structural impurities and chemical contaminants (RLCs, CEI remnants) hinder performance and regeneration by impeding lithium diffusion and structural reconstruction. Therefore, simultaneously repairing the bulk structure while engineering a stable, functional surface is highly attractive. Generally, surface modification during upcycling leverages the high temperatures or reactive environments inherent in regeneration techniques (e.g., molten salt or solid-state sintering) to form or deposit the desired surface phase in situ or facilitate its reaction with the degraded surface. Such an integrated approach aims to passivate reactive surfaces, remove contaminants, enhance ionic/electronic transport, and ultimately improve the overall stability and performance of the upcycled material.

A range of metal oxides have been explored for their protective effects in upcycling NRLOs. These oxides, such as MoO₃,^130,131 TiO₂,¹³² La₂O₃,¹³³ tend to react with surface residues or the NRLO surface itself to form Li-contained surface layers, permitting improved interfacial charge transfer and stability. For example, Tong et al.¹³² demonstrated the in situ formation of Li₂TiO₃ coating during molten salt regeneration of NCM523 using a TiO₂ additive. The modification boosted charge transfer and inhibited TM dissolution. Recently, Zhou and colleagues¹³⁴ proposed a channel-assisted regeneration strategy utilizing waste spinel LiMn₂O₄ (LMO). This approach was applied to spent NCM523 and extended to NCM811, reconstructing their surfaces into a 3-dimensional (3D) structure. This surface reconstruction facilitated efficient replenishment of lithium into the particle lattice, and the resulting 3D ion channels improved fast-charging performance. For NCM523, this resulted in regenerated material achieving a capacity retention rate of 87.9% after 500 cycles at 10C, with overall electrochemical performance significantly outperforming commercial materials (Fig. 10a). Oxide-type surface reconstruction is also applicable to humidity-degraded NRLO. For example, a La salt, La(NO₃)₃·6H₂O, was proposed to scavenge residual Li/NiO impurities from degraded NCM811 to form a conductive La₄NiLiO₈ coating surfaces, allowing improved Li⁺ transport and durability.¹³⁵ Recently, Chen et al.¹³⁶ developed an Nb-assisted eutectic upcycling method for NCM523. During this process, Nb₂O₅ reacted with the degraded NCM to form a Li₃Ni₂NbO₆ epitaxial layer. This layer effectively filled the grain boundaries, helping to maintain the microstructure and significantly enhancing structural recovery.

Beyond oxide coatings, fluorides,¹³⁷ phosphates,¹³⁸ borates,^139,140 and silicates¹⁴¹ are also explored as protective barriers or functional layers during upcycling. Guo et al.¹³⁷ combined hydrothermal and molten-salt annealing to regenerate the degraded NCM523, where LiF derived from CEI and PVDF binder in situ coated on the surface of the cathode (Fig. 10b). Ryu et al.¹³⁸ treated spent NCM811 with (NH₄)₂HPO₄ to form a nanoscale Li₃PO₄ coating from residual surface lithium, which protected against environmental degradation and parasitic reactions while improving Li⁺ migration. In another study. Zhou's team¹⁴⁰ developed LiCo_0.5Mn_0.5BO₃ (LCMB) channels applied to degraded NCM523, aiming to clear CEI and provide low-barrier pathways for faster relithiation (Fig. 10c). A Li₄SiO₄/SiO₂ coating layered was used as structural regulators and protective layers against electrolyte corrosion for air-degraded NCM83.¹⁴¹

Despite demonstrated benefits, surface modification during NRLO upcycling remains challenging. Precisely controlling coating thickness, composition, and crystallinity during high-temperature processing is difficult, even when modifying pristine CAMs.^142,143 The achievement of uniform and conformal coverage on particles with varied degradation levels becomes considerably more complex due to the unpredictable nature of variable residual impurities. Scalability further presents a significant concern. Addressing these multifaceted difficulties necessitates focused future investigation aimed at developing novel materials (especially those designed to interact favorably with or potentially utilize residual contaminants), optimizing in situ formation mechanisms and process control, enhancing coverage uniformity and adhesion on complex substrates, and devising cost-effective, scalable application techniques. Such advancements are vital for the practical implementation of surface modification as a key upcycling strategy for NRLOs.

These upcycling efforts extend beyond simple regeneration by incorporating intentional modifications to compositional tuning, morphological control, lattice doping, and surface engineering. Compositional upcycling focuses on adjusting the metal ratios, primarily increasing nickel content, to upgrade the material feedstock. Morphological upcycling seeks to transform particle shape, often targeting robust single-crystal structures from degraded polycrystals. Lattice doping introduces beneficial elements into the crystal structure to enhance properties, including exploring the potential of controlled impurity utilization. Surface modification aims to engineer protective or functional layers on the particle surface to improve interfacial stability and ion transport. Notably, these distinct upcycling strategies are often applied in combination to achieve synergistic outcomes. While common challenges include achieving precise control over the introduced modifications, effectively managing the variable impurities inherent in spent materials, and ensuring the scalability and economic viability of the processes. Despite these complexities, upcycling offers a compelling pathway for value addition in LIB recycling by leveraging aspects of the degraded material itself. Table 2 provides a detailed summary of reported research across these various upcycling strategies and their outcomes.

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While direct regeneration and upcycling methods inherently offer distinct advantages over traditional pyrometallurgical and hydrometallurgical routes—including their non-destructive nature, streamlined processes, reduced costs, and enhanced environmental profiles—a more granular comparison among these direct strategies themselves is necessary for guiding future development. To facilitate a comprehensive understanding, Table 3 provides a systematic comparison of the diverse direct recycling strategies detailed in Sections 3 and 4. This table meticulously outlines the key advantages and disadvantages intrinsic to each direct regeneration and upcycling technique. Crucially, it explicitly links each method to the main NRLO degradation mechanisms (as elucidated in Section 2) that it is most adept at addressing or the specific objective it aims to achieve, thereby enabling readers to discern the optimal approach for various degradation scenarios.

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Complementing this, Fig. 11 presents radar charts visually comparing these direct recycling methods across five key practical considerations: energy consumption, pollution emission, regeneration efficiency, scalability, and cost. It is important to note that for liquid-processing regeneration methods like hydro/solvothermal, chemical, and electrochemical approaches, the actual energy use, emission, and overall cost could be substantially higher than initially perceived due to the necessity of a subsequent re-annealing step. For the upcycling methodologies, while generally entailing higher cost and energy input due to the incorporation of additional chemicals or extended processes, they typically yield upcycled NRLOs that exhibit superior electrochemical performance and consequently, higher intrinsic value.

5. Summary and outlook

The escalating global demand for LIBs, particularly those employing high-energy NRLOs, underscores the urgent need for sustainable end-of-life management. As detailed in this review, NRLOs are intrinsically susceptible to multifaceted degradation pathways during cycling and storage, including phase deformation, stoichiometric imbalance, surface contamination, and morphological damage. These degradation phenomena not only compromise electrochemical performance but also present significant challenges for effective recycling.

Direct regeneration has emerged as a promising closed-loop alternative that aims to restore the properties of spent CAMs by repairing chemical and structural degradations to recover original performance. This review systematically examined contemporary methods, including solid-state sintering, hydro/solvothermal treatment, molten-salt fluxing, chemical lithiation, and electrochemical lithiation, in regenerating NRLOs. Solid-state sintering is an established method, adaptable but facing kinetic limitations. Hydro/solvothermal and molten-salt methods leverage liquid environments for enhanced mass transport but often require complex multi-step processes involving post-treatment for solvent/salt removal. Lower-temperature chemical and electrochemical routes offer energy-saving potential but are currently primarily limited to lithium replenishment, often necessitating subsequent thermal annealing for full structural repair.

Extending beyond simple restoration, direct upcycling strategies target enhanced electrochemical performance by employing intentional modifications to the spent material. These approaches build upon regeneration techniques by incorporating strategic modifications such as compositional upgrades, morphological control, lattice doping, and surface engineering. These strategies are often applied in combination to achieve synergistic outcomes, leveraging aspects of the degraded material itself to impart improved or new functionalities and present a compelling pathway for value addition in LIB recycling.

While significant progress has been made in developing these direct recycling methods for NRLOs, their practical implementation and widespread adoption face considerable challenges. These include the effective and scalable management of varied impurities from reclaimed black mass, achieving precise control over the restoration or modification process amidst feedstock variability, ensuring scalability, and establishing economic viability. A comprehensive visual overview of the key stages and concepts discussed throughout this review is provided in Fig. 12. The diagram illustrates the journey from NRLO-based spent LIBs through the two primary direct recycling pathways: direct regeneration, aimed at restoring original performance, and direct upcycling, targeting enhanced performance. The figure also emphasizes the overarching challenges, such as feedstock variability, impurity stubbornness, process complexity, and scalability and economic concerns.

Building upon this comprehensive understanding and the challenges highlighted, the outlook ahead focuses on key areas for advancing NRLO direct recycling towards industrial reality. Future directions should prioritize:

(1) Efficient feedstock treatment: a significant challenge currently limiting the industrialization of direct regeneration and upcycling is the difficulty in obtaining a clean and consistent feedstock from commercial spent batteries. Unlike controlled laboratory settings, real-world black mass collected from diverse sources is often highly contaminated. This contamination manifests in two primary forms: cross-contamination from varying types of cathode materials within the same LIB stream (e.g., mixtures of LFP, NCM111, NCM523, NCM622, and NCM811) or even different battery chemistries entirely (e.g., lead-acid and nickel–cadmium batteries mixed alongside LIBs). Additionally, internal contamination stems from residual battery components like aluminum foil, binder, conductive carbon, polymer separators, residual electrolyte, anode components, and shell parts. Such mixed and impure feedstocks pose substantial obstacles, as direct recycling processes typically require a relatively pure and compositionally uniform input to effectively restore or upgrade the material.

Therefore, a crucial future direction involves the development of efficient and scalable pretreatment methods specifically designed to handle these complex, mixed waste streams. This includes exploring sophisticated sorting technologies to address cross-contamination, as well as delicate physical and chemical separation processes (e.g., magnetic separation, froth flotation, solvent immersion, molten salt melting, incineration) for internal component contaminants, which have been summarized elsewhere.^150,151 Critically, the selection of pretreatment methods should ensure the preservation of the cathode structure, a fundamental requirement for direct recycling approaches.

(2) Innovative recycling methods: future efforts should focus on designing processes that intelligently leverage specific characteristics of the degraded feedstock and significantly improve the efficiency of key steps, such as achieving rapid and homogeneous relithiation and complete phase recovery. In terms of morphological control, while single-crystallization is extensively reported for regenerated NRLOs, the drawbacks of extended diffusion pathways and internal stress accumulation cannot be overlooked.^55,152 Innovative methods must therefore aim for optimized particle morphologies, meticulously controlling crystal growth and minimizing adverse crystal defects to ensure the long-term reliability of reproduced materials.

(3) Development of computational tools: applying advanced computational tools, particularly ML and artificial intelligence (AI), holds significant potential for accelerating advancements in direct recycling.¹⁵³ Unlike conventional computational methods, ML/AI excels at analyzing vast, high-dimensional datasets and identifying complex, non-linear relationships.¹⁵⁴ This capability is particularly attractive for simulating degraded NRLOs with heterogeneous and compositionally varied degraded phases across multiple dimensions, enabling significantly faster screening of materials and optimization of processes from atomic-level interactions to macroscopic parameters.

The effectiveness of these tools, however, is critically dependent on the availability of reliable, comprehensive databases for training. These databases, ideally integrating both experimental data and reliable simulation data (e.g., from density functional theory (DFT) or molecular dynamics (MD) simulations), are essential for building robust models. When trained with such high-quality data, ML/AI techniques can effectively predict material behavior, forecast optimal recycling parameters, and even guide the de novo design of novel, highly efficient recycling routes and materials. For instance, ML has been successfully applied to optimize molten salt composition design by mapping properties like melting point, viscosity, and ion diffusion. This capability allows for accelerated screening of optimal salt systems and more precise control over crystal growth processes, which is crucial for complex phase reconstruction from degraded materials.

(4) Environmental friendliness Ensuring sustainability is critical throughout the entire direct recycling chain. This requires minimizing energy consumption, reducing chemical waste generation, and developing eco-friendly reagents and processes at every step.

(5) Scalability and economic viability: translating laboratory successes to industrial scale is a significant challenge. This necessitates designing cost-effective processes, optimizing resource utilization, improving process throughput, and conducting comprehensive techno-economic assessments to demonstrate commercial viability.

Advancing these technologies requires concerted efforts encompassing fundamental materials science, chemical engineering, and interdisciplinary collaboration. Ultimately, realizing robust and economically viable direct regeneration and upcycling of NRLOs is essential for closing the loop in the battery supply chain, enabling resource security and contributing to a sustainable energy future.

Author contributions

Gui Chu: methodology, data curation, visualization, writing – original draft. Yu Huang: data curation, formal analysis, visualization. William Hawker: validation, formal analysis, writing – review & editing. Lianzhou Wang: conceptualization, validation, writing – review & editing. Xiaobo Zhu: conceptualization, supervision, writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

This manuscript is a review article. No new data, software, or code were generated in this study. All information discussed is sourced from previously published research, which is cited accordingly.

Acknowledgements

The authors acknowledge the financial support from the National Natural Science Foundation of China (52202210), the Natural Science Foundation of Hunan Province (2024JJ5024), and the Australian Research Council (FL190100139).

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