A Review on the Role of Crosslinked Polymers in Renewable Energy: Complex Network Analysis of Innovations in Sustainability

Abstract

As the global push for renewable energy intensifies, the materials used in the generation, transmission, and storage of renewable energy systems have come under scrutiny due to their environmental impact. In particular, crosslinked polymers are extensively utilized in these systems because of their excellent thermal, mechanical, and electrical properties. However, their non-recyclable nature and significant waste generation at the end of their service life present severe sustainability challenges. This review employs a citation network-based methodology to analyze the role of crosslinked polymers in renewable energy systems, with a focus mainly on two critical applications: (1) production, specifically in the manufacturing of wind turbine blades; and (2) transmission, where they are integral to high-voltage cable insulation. Our complex network analysis reveals the major themes within the field of sustainability, providing a structured approach to understanding the lifecycle challenges of crosslinked polymers. The first part explores the primary polymers used, their typical lifespans, and the environmental burden of generated waste. We then describe both traditional recycling strategies and innovative approaches, such as supercritical water processing and thermoplasticizing technologies, which offer potential solutions to mitigate these impacts. Finally, we highlight emerging reprocessable materials, including vitrimers, ionomers, and specialty thermoplastic alternatives, which provide recyclability while maintaining performance. This comprehensive assessment emphasizes the urgent need for innovation in polymer science to achieve a circular economy for renewable energy systems.

1. Introduction

According to the World Bank, energy is the pillar of a modern economy: it enables investments, innovations, and new industries that drive employment, inclusive growth, and shared prosperity on a habitable planet [1]. This makes energy availability strategic for human development, as fossil fuels have demonstrated throughout the 20th century. However, their use has brought significant environmental consequences, particularly an increase in greenhouse gas concentrations in the atmosphere, such as CO₂ and NO_x. According to the UN, these gases are primarily responsible for climate change, with fuel combustion accounting for over 75% of global greenhouse gas emissions and nearly 90% of all CO₂ emissions [2]. This fact highlights the need to reduce emissions by half by 2030 and achieve net zero by 2050. As a result, alternatives to renewable energy sources without greenhouse gas emissions have been sought. Two options have garnered significant interest due to their abundance: solar energy, as the Earth’s atmosphere receives 1360 W/m² constantly [3], and wind energy, which derives from the former and has a higher theoretical efficiency factor than solar photovoltaics, with the Betz law predicting a maximum conversion of 59% [4].

Significant advances have been achieved in the field during the last decades, resulting in an increasing share of world’s demand for energy being met by renewables, with photovoltaics and wind accounting for over 95% of all the renewables combined, according to the report released by the International Energy Agency, Renewables 2024 [5]. Moreover, figures are expected to continue rising over the next years thanks to the goals set to triple the renewable energy production by 2030 [6]. Although these numbers are promising, integral systems for renewable energy generation and transmission are not exempt from environmental challenges. For example, wind turbine blades are typically sent to landfills after use, with materials such as crosslinked epoxy resins and glass fibers contributing to long-term non-degradable waste [7]. Composite materials, especially laminated composites, pose difficulties for recycling: while recycling technologies are usually the most environmentally friendly solution, they are only practical if the value of the recovered raw materials exceeds the cost of the recycling process [8]. Nevertheless, many of these processes can become more attractive (even from the economic point of view) in the short or medium term. A growing body of government regulations and incentives for cleaner and more environmentally friendly means of production is already taking form in several countries and regions, endorsed by the Sustainable Development Goals set by the United Nations through the 2030 Agenda for Sustainable Development. These Sustainable Development Goals (SDGs) propose the creation of “Affordable and Clean Energy”, in line with “Climate Actions” (SDG 7 and SDG 13, respectively), seeking to reduce dependence on fossil fuels and greenhouse gas emissions, as well as “Responsible Production and Consumption” (SDG 7), promoting the efficient use of resources through the reuse and recycling of materials. This, in addition to promoting the production of clean energy, drives the development of new low-impact technologies in the field of renewable energy: recycling methods for composites and new reusable polymeric materials to increase the environmental sustainability of energy production and transmission.

As indicated above, renewable energy generation also requires transmission systems that often rely on high-voltage alternating current (HVAC) and high-voltage direct current (HVDC) lines. Unlike traditional generation, renewable energy is often generated in locations that are geographically far from urban centers, making these lines much longer. These systems, while efficient at reducing transmission losses, make extensive use of materials such as crosslinked polyethylene for insulation, which, despite its excellent electrical and thermal properties, is not recyclable and contributes to end-of-life waste in large-scale projects such as Desertec or European SuperGrid, where power generation would be located thousands of kilometers away from consumption centers [9,10].

Both renewable energy systems and the transmission lines that support them highlight a recurring issue: the widespread use of crosslinked polymers. These materials, while offering superior mechanical and thermal resistance, are inherently non-reusable due to their three-dimensional structures based on permanent covalent bonds. Addressing this problem has led to significant research efforts aimed at developing high-performance materials that can be recycled or reused. In this review, we will examine the use of crosslinked polymers, which are not easily recyclable or reusable, in various aspects of renewable energy systems. Specifically, the review will be structured around two key areas: (1) production, focusing on the use of crosslinked polymers in the manufacture of wind turbine blades; and (2) transmission, where crosslinked polymers are integral to high-voltage cable insulation. For each of these areas, we will explore the primary uses of crosslinked polymers, the main types of crosslinked materials employed, their lifespan, and the estimated waste generated at the end of their lifecycle. Furthermore, we will discuss strategies aimed at mitigating the environmental impact of these materials, including replacing traditional crosslinked polymers with reprocessable alternatives, such as vitrimers and ionomers, and the development of recycling methods for traditional crosslinked materials.

This review aims to highlight both the challenges and the advancements in polymer technologies, as they relate to reducing the ecological footprint of renewable energy production, transmission, and storage systems. We employ a citation network-based methodology to map the landscape of sustainability challenges and innovations related to crosslinked polymers in renewable energy systems. By analyzing interrelations among research topics, this approach reveals thematic clusters, providing a structured understanding of lifecycle challenges, recycling strategies, and emerging alternatives. These clusters highlight critical applications, such as crosslinked polymers in wind turbine blade production and high-voltage cable insulation, emphasizing the need for material innovations to achieve circularity in renewable energy systems.

2. Reviewing Methodology

The complex network-based methodology employed in this review provides a structured method for analyzing the scientific landscape surrounding sustainable crosslinked polymer technologies [11,12,13]. It is important to remark that even when dealing with a relatively narrow topic, namely sustainable polymeric materials for renewable energy harvesting and transport, it is impossible to exhaustively review all of the existing literature. Therefore, informatics tools such as complex citation networks and rankings of the most relevant articles in each area appear as highly useful instruments [11,12]. Our method involves three primary steps: (1) constructing a citation network to approximate the underlying knowledge map (i.e., the current collective scientific understanding of the selected topic), (2) identifying key research themes through clustering analysis, and (3) interpreting the topics these clusters represent within the context of renewable energy application.

2.1. Citation Network as an Approximated Knowledge Map

We propose to provide a glimpse into the knowledge map of renewable energy by constructing a complex network of citations around recent publications in the field. As mentioned, this network is generated from a survey of the current literature on the field and can help visualize some of the most relevant topics and their interrelations (see Figure 1). The network shows several large clusters that include, to a large extent, all the relevant topics we intend to cover in this review. Many major nodes are connected to the central large cluster, with those related to fiber-reinforced polymers and composite recycling being the most important ones. These also show multiple links to composites for wind blades, as logically expected, and to biobased and self-healing materials. The latter works as functions as a hub, connecting with the cluster related to power transmission. Other lateral themes, such as thermoplastics: 3D-printed continuous fiber composites (such as PLA, PEEK, etc.), pultrusion methods, void characterization, and recycling/remanufacturing appear in the network, but they are more loosely linked to the core subjects. Within this cluster, radical polymerization can be thought of as one of the main connecting topics, since it is the most widely used crosslinking method to obtain crosslinked polyethylene (XLPE), with dicumyl peroxide (DCP) as the initiator. Nevertheless, besides its non-recyclability, an important downside of the DCP-initiated crosslinking is the volatile and harmful species produced by the decomposition of DCP. These species must be eliminated from the XLPE in order to prevent the electrical conductivity from increasing and the dielectric breakdown threshold from decreasing. Since degassing involves a time-consuming and costly process, this is a major factor limiting the usage of XLPE for HV cable insulation [14,15].

2.2. Network Construction Methodology

To study the scientific landscape in sustainable crosslinked polymer technologies, a systematic methodology was employed to construct and analyze a complex bibliographic network. The workflow was divided into three main stages: (a) downloading citations and constructing subnetworks, (b) merging these subnetworks into a comprehensive bibliographic network, and (c) calculating relevant network parameters to identify trends and contributions.

A set of seed publications was identified based on their relevance to the scope of this review, such as the recycling of thermosetting polymers, vitrimers, ionomers, and crosslinked polyethylene, the number of citations and date (see also Supplementary Information Section). The Digital Object Identifier (DOI) of each seed publication was used as an entry point to extract metadata and references from crossref.org. The extracted metadata included titles, abstracts, authors, and citations. For each seed publication, a recursive function was implemented to traverse its reference network to a previously defined depth (in this work, we employed 2 levels). This ensured the inclusion of both direct and secondary references, allowing for a holistic representation of the citation network surrounding each seed topic. Directed edges were added to represent citation relationships. The citation network in this review was obtained by using nine articles as seeds and tracking their references (first neighbors) and the references of those references (second neighbors). A schematic illustration of the subnetwork creation is shown in Figure 2.

Individual subnetworks were then combined into a unified network. Duplicate entries were removed to ensure that the nodes and edges were unique. The final network, amounting to 26,559 nodes (publications) and 41,449 edges (citations), was analyzed to identify major themes and key publications. The robustness of the method was tested by comparing the obtained network with a different one using six articles as seeds instead, which resulted in a similar network, essentially covering the same topics (see Supplementary Information Section for details). Metrics such as betweenness centrality and PageRank scores were computed to locate influential nodes and thematic clusters within the network [16,17].

To better understand the thematic content of these major clusters, they were filtered using PageRank scores and betweenness centrality to identify their most influential nodes (publications). In graph theory, betweenness centrality is a measure of centrality based on shortest paths (Figure 3). For every pair of vertices in a connected graph, there exists at least one shortest path between the vertices, such that the number of edges that the path passes through is minimized. The betweenness centrality for each node is the number of these shortest paths that pass through it (see [18]). Betweenness centrality, therefore, represents the degree to which nodes lie between others. A node with higher betweenness centrality has more control over the network, because more information passes through it. On the other hand, PageRank is a measure of the importance of nodes within a network, based on the structure of incoming links (see Figure 4 and [19]). It was originally developed to rank web pages in search engine results, and the algorithm operates under the assumption that more important nodes are likely to receive more links from other nodes. PageRank assigns a probability distribution representing the likelihood that a random walker would land on a particular node at any given time. This probability is calculated iteratively, taking into account both the number and the quality of links, with links from highly ranked nodes contributing more to the score. Thus, nodes with higher PageRank values are considered more central or influential within the network.

In addition, publication titles and abstracts within these filtered clusters were examined using word frequency analysis to extract key terms. For each cluster, word clouds were generated to visually represent the content and highlight recurring terms. An example of the results for reinforced fibers clusters can be observed in Figure 5, where natural fibers and glass and carbon fibers seem to prevail among other topics, such as experimental characterization and matrix composition.

The network construction was implemented in Python 3.7, using networkx for graph management. Network calculations, clustering analysis, and visualization were performed in Gephi 0.9. This method provides a comprehensive and systematic way to identify critical contributions and interconnections within the field, enabling a more objective discussion of current trends and future directions.

3. Discussion

In this section, we present the findings from our analysis, offering a holistic view of the role of advanced materials in renewable energy systems. We begin with a general overview of the field, highlighting the major thematic clusters identified through network analysis and their implications for sustainability and performance in renewable energy applications (Section 3.1). This is followed by a focused discussion on the generation of renewable energy, particularly addressing the challenges posed by non-recyclable materials in wind turbine blades and the emergence of vitrimer-based solutions (Section 3.2). We then explore the transmission of power, emphasizing the dominance of XLPE in high-voltage applications and the potential of recyclable alternatives and life span extension alternatives (Section 3.3). Finally, we address other secondary applications, such as the use of crosslinked polymers in lithium-ion batteries and emerging strategies for enhancing their recyclability and performance (Section 3.4).

3.1. General Overview

The identified clusters were analyzed to draw connections between material innovations and their practical implications in renewable energy systems. In particular, we identified the topics within the different clusters, and the nodes with higher PageRank and betweenness centrality to assess the amount of information that passes through them. For instance, while one cluster emphasizes the environmental challenges of non-recyclable wind turbine blades, another one focuses on the development of vitrimer-based alternatives. On the other hand, when analyzing the self-healing cluster, we observe that there are several new materials being developed (such as vitrimers, ionomers, and novel copolymers) that could find applications in energy harvesting or transmission devices. We also observe advanced recycling strategies for thermosets, which are emerging as viable alternatives to reduce the ecological footprint of energy generation and transmission.

In the case of wind turbines, the main problem lies in the waste generated by the turbine blades. These are mainly made of composite materials based on a mesh of glass fibers (GF) or the more resistant carbon fibers (CFs) and a thermosetting polymer as the matrix. Initially, polyester resins fulfilled this role, but with the development of large and extra-large wind turbines, epoxy resins replaced polyester and are now the most widely used matrices for wind blade composites [20]. Tazi et al. indicate, based on the analysis of reports from companies such as Vestas, that the end-of-life treatment of the blades is divided between 50% landfill and 50% incineration [21]. This is due to the nature of the permanent covalent networks of the thermosetting polymers used in the composites, which prevents both their easy recycling and the recovery of glass fibers and the even more expensive and more energy-demanding carbon fibers [7].

Whilst being a negative aspect, the non-recyclability of thermosetting matrices is also an incentive to develop recycling routes or reprocessable resins that allow for their recovery and can provide future benefits. Given this context, various ways of reusing and recycling wind blades made of thermosetting composite materials have been proposed. The reuse of the blades is limited by their geometry and dimensions [22]. In the case of recycling, various techniques, such as pyrolysis and solvolysis, have been studied to recover the initial monomers, and especially the fibers, but all of them involve intensive chemical and energy consumption processes, making the recycling process less environmentally friendly. Additionally, it is very likely that these processes will cause damage to the fibers, reducing their value and their options for use [23].

Regarding reprocessable materials, several strategies have been proposed, many of which are based on epoxy resins that can be recycled but require expensive chemical digestion processes [24]. These methods seek to recover the reinforcing fibers but still at a cost in inputs and energy that makes these approaches industrially unattractive. More recent options seek to use covalently adaptable networks (CANs) [25] or infusible thermoplastic resins [26], which allow for thermoforming and recycling with fiber separation, without the need for chemical processes. Emerging materials such as polyimine networks, explored by Taynton et al. [27,28], are driving the development of fully recyclable carbon fiber composites. These malleable networks enable repairability and recyclability without compromising the mechanical properties of the fibers, representing a significant shift toward sustainable composite materials for wind and solar energy systems. Yu et al. [29] further advance this area with thermoset composites that achieve near 100% recyclability. Their work demonstrates that CAN allow for efficient fiber recovery while maintaining the structural integrity of the recycled material. These innovations therefore address one of the key barriers in traditional thermoset recycling (the irreversibility of the polymer network), which has historically limited recyclability. It must be kept in mind, however, that there is still a way to go before these materials can be successfully applied to large scale projects such as wind blades. One of the most relevant and challenging issues to overcome is the delivery of the external stimulus to activate the exchange reactions. This constitutes a very active research field, which will be addressed in the following sections.

Pimenta’s review highlights the growing market potential for recycled carbon fiber-reinforced polymers (CFRPs), particularly as their use expands in structural applications such as wind turbine blades [30]. Similarly, Oliveux et al. provided a comprehensive review of the current recycling technologies for fiber-reinforced polymers, emphasizing mechanical, chemical, and thermal processes [31]. Their work outlines the progress made in reducing environmental impacts through innovative recycling methods such as solvolysis, which offers high recovery efficiency for both fibers and matrices. However, they also underscore that high costs and energy demands still limit its large-scale adoption. More recently, De Fazio et al. [32] provided an updated revision of the traditional methods for recycling thermoset-based fiber composites or for recovering the fibers, still showing concerns about the high energy consumption and environmental impact of chemical and thermal recycling strategies. Deterioration of GF properties, or the shorter fibers resulting from mechanical methods (hammer milling, shredding, milling, and grinding), which cannot be used in the same way as the original ones, are also an issue that has not been properly tackled. Mishnaevsky’s overview of materials for wind turbine blades illustrates how the increasing use of carbon fiber composites, driven by the need for larger, lighter blades, exacerbates the end-of-life disposal challenge [20]. This shift towards more robust materials highlights the urgency of developing more effective recycling strategies. Current solutions, such as vitrimer-based matrices and advanced solvolysis processes, are promising but require further optimization to balance performance with environmental sustainability.

In the case of power transmission lines, XLPE is the dominant material for HVDC and HVAC cable insulation, due to its excellent thermal and electrical properties, making it ideal for long-distance power transmission [25]. However, its non-recyclable nature has become a major sustainability challenge. The three-dimensional covalent bond structure of XLPE renders it impossible to melt and reuse, leading to significant amounts of waste, particularly as renewable energy infrastructures continue to scale up globally [26]. The modification of these materials is more complex in HVDC lines due to the nature of DC power transmission: modifications must not affect the electrical conductivity of the insulating material, as this may lead to dielectric failure problems [33]. A clear example of this occurred in the first attempts at radical crosslinking with dicumyl peroxide in XLPE: this reaction gives rise to undesired volatile by-products that tend to increase electrical conductivity, and whose removal process had to be more exhaustive than that previously used for HVAC lines in order to obtain successful XLPE-insulated HVDC lines [33,34]. This presents a barrier to any modification in the insulation of HVDC lines and is a crucial factor in the transmission of power generated from renewable sources, which requires very long transmission lines where the use of HVDC is economically more convenient than that of HVAC lines [34]. As Figure 6 shows, HVDC cables make use of a higher volumetric proportion of XLPE than HVAC cables for transmitting similar amounts of power, so HVDC cables have less recyclable material per unit volume than the HVAC ones. Efforts to improve the performance of XLPE include incorporating nanocomposite modifications, which can mitigate issues such as partial discharges and space charge accumulation, extending its operational life [35].

Despite these advancements, XLPE still presents an environmental burden due to its lack of recyclability. Moreover, its degradation due to charge generation and electrical stress remains a concern. Over time, partial discharges can lead to material degradation, reducing the lifespan of HVDC cables. Addressing these challenges requires the development of more durable and recyclable materials capable of withstanding long-term electrical and thermal stresses [36]. Alternative materials such as isotactic polypropylene (i-PP) [37] and propylene-ethylene copolymers [38] are also being explored, offering enhanced mechanical properties at higher temperatures while maintaining strong dielectric performance. However, these alternatives do not fully solve the recyclability challenge [26]. Polyethylene-based ionomers can be a more sustainable option, exhibiting similar mechanical and dielectric properties to XLPE, but being melt-processable and recyclable [39]. Ionomers avoid the production of harmful byproducts during synthesis and offer lower dielectric loss and DC electrical conductivity, positioning them as a promising alternative for high-voltage cable insulation with both performance and sustainability benefits.

Figure 6. Examples of structures of HV submarine cables rated for 200 MW, manufactured by Sumitomo Electric (Osaka, Japan): (A) 3-core HVAC 110 kV cable; outer diameter: 193 mm; (B) +/−160 kV HVDC cable; outer diameter: 111 mm. Reproduced with permission from [40,41].

Figure 6. Examples of structures of HV submarine cables rated for 200 MW, manufactured by Sumitomo Electric (Osaka, Japan): (A) 3-core HVAC 110 kV cable; outer diameter: 193 mm; (B) +/−160 kV HVDC cable; outer diameter: 111 mm. Reproduced with permission from [40,41].

3.2. Generation

Regarding energy generation, particularly from renewable sources, wind and solar power have garnered significant attention due to their potential to reduce greenhouse gas emissions. However, the environmental footprint of these technologies extends beyond their operational phase, especially during end-of-life management. Wind turbine blades, predominantly composed of crosslinked polymers in glass or carbon fiber-reinforced polymer composites (GFRPs or CFRPs, respectively), pose significant recycling challenges due to their non-reprocessable nature, contributing to long-term waste accumulation.

Wind Turbine Blades

The issue of the final disposal of fiber-reinforced thermosets in wind blades has gained urgency in the last decade, as the first generation of wind blades reached the end of their service life and started to be dismantled and replaced by newer (often larger) ones. These blades, primarily made of glass fiber-reinforced polymers, present a significant end-of-life challenge due to their non-recyclable nature [42]. It is estimated that 43 million tons of blade waste will accumulate globally by 2050, with China, Europe, and the United States being the largest contributors [42]. This impending waste problem highlights the urgent need for the development of sustainable recycling strategies for composite materials [8]. Several European countries will ban the landfilling of wind turbine blades within a few years, and many US states have introduced (though not yet passed) laws requiring wind blade manufacturers to be jointly responsible for the final disposal of turbine blades, or that ban landfilling altogether [43,44].

Although 80% of the materials in wind turbines can be recycled [45], the blades remain a significant challenge due to their composite structure, comprising thermosetting matrices. Addressing this challenge requires a holistic approach that considers the entire value chain, from design to disposal, integrating circular economy principles and lifecycle engineering into the production process. This involves designing blades with end-of-life recovery in mind, utilizing reprocessable materials, and developing new recycling technologies to handle complex composite structures [45,46]. Figure 7 shows a schematic picture of the current end-of-life processes for wind blades.

Repurposing wind blades for new structural or architectural applications is a promising solution [22,44,45,46]. In particular, reusing them without the need for significant reprocessing considerably reduces environmental impact throughout their lifecycle. The robust design of the blades to withstand harsh conditions makes them suitable for various urban structures, while their geometric limitations and the associated logistical challenges prevent the entire mass of wind blades from being repurposed. Moreover, with the increasing number of wind-blades reaching their end of life, a growing amount of them will need to be treated by some recycling method, either an existing one or another that has yet to be developed. Flat sections are the most interesting ones and can be used in building roofs, house cladding, parts of bridges, urban furniture, or road signs. For example, some of the proposed solutions to save limitations are related to the use of digital reconstruction to design 3D structures using wind blade geometry [47]. Figure 8 illustrates innovative architectural applications for repurposed wind blades, showcasing their potential beyond conventional recycling methods.

Regarding the methods for recycling wind blades, they can be divided into four groups according to the main processes involved in the treatment: mechanical, thermal, electrical, and chemical. As shown in Figure 7, the application of each of them is mainly conditioned by the type of fibers used in the construction of the wind blades.

Mechanical processing is currently the fastest, cheapest, and best understood way of processing wind blades [22]. It involves grinding the blades into smaller particles for their reuse in lower-grade applications, and, while cost-effective, it results in downcycling (therefore limiting the quality and application of recovered materials) [22]. Processing close to wind farms is critical for transport efficiency and is one of the key points defining cost-effectiveness. If processed into small segments of glass fiber strands, the obtained materials can be used to reinforce plastics (including injection molding), as insulation in drywalls, as strength additives in concrete, among other construction applications [22]. If ground finer into a powder, there are many other opportunities where coal ash is currently employed, such as drywall, structural fillers, and cement kilns [22]. In the case of cement kilns, the use of GFRP waste as an additive for the manufacture of Portland cement or alumina cement heavily depends on economic viability and market acceptance, and is still under study [50,51]. Mechanical methods are often economically practical for GFRP. On the other hand, the recovery of CFs from CFRPs for further utilization is justified by the higher value of the recovered fibers and has to be done through more sophisticated strategies [52,53].

Regarding thermal processes, techniques such as pyrolysis and microwave-assisted pyrolysis can recover fibers by decomposing the polymer matrix (Figure 9). It should be noted that the most basic thermal treatment is the utilization of the materials as fuel (as indicated above in cement-kiln recycling). However, to recover fibers, processes such as pyrolysis are a more feasible option [50,52,54]. Pyrolysis uses heat in an inert atmosphere, moderate temperatures, and atmospheric pressures to separate the different materials that constitute the wind blades, i.e., polymeric materials are burned, then fibers, metals, and fillers are separated. In the case of carbon fibers (CFs), heat treatment often fails to completely remove the resin from the fibers and can also negatively affect the fiber properties. While this process is still being improved to recover CFs with high property retention, it is unlikely to gain general acceptance due to the use of hazardous chemicals, as well as excessive energy and capital costs [52].

Microwave pyrolysis appears as a very promising technique for recycling large quantities of wind turbine blade waste [53]. The use of microwaves on these low thermal conductivity materials produces homogeneous heating, which is impossible to achieve in common thermal ovens, and therefore exposes the fibers to lower temperatures compared to pyrolysis. There are several variants of this process: (i) Ren et al. [55] worked on a method to recover CFs using microwave pyrolysis and a subsequent oxidative treatment to clean the CFs from any resin residues, where the recovered fibers retain more than 96% of the tensile modulus with respect to the original fibers; (ii) Rani et al. [56] worked on microwave-assisted pyrolysis using a solvent system composed of hydrogen peroxide and acetic acid, with different solvent–composite ratios and microwave exposure durations; (iii) D. Zhang et al. [57] further investigated this type of processing by adding silicon carbide (SiC) particles as a microwave absorber, looking for optimal working conditions on an industrial scale at different microwave powers, wind blade/SiC ratios and SiC particle sizes; (iv) L. Zhang et al. [58] employed graphene supported on a porous alumina fiber material to generate plasma under microwave irradiation in a N₂ atmosphere to improve the graphitization of the fibers. However, further research is still needed to optimize the process parameters and understand the effects of recycling on the mechanical properties of the recovered fibers. It can be said that, of all the recycling methods being currently developed, this is the one with the lowest technological readiness level (TRL): approximately 4–5 [43]. Notwithstanding, it has the potential to be the method with the lowest energy consumption (10 MJ/kg raw material) [59].

Another method that allows fibers to be recovered without being exposed to high temperatures is high-voltage fragmentation. This method uses electrical pulses to disintegrate the materials by taking advantage of the presence of a multi-phases structure to separate them at their boundaries. Compared to mechanical recycling, high-voltage fragmentation produces longer and cleaner fibers but requires higher energy inputs [59,60].

Finally, the chemical methods allow for a more complete recovery of the components in the composites, since they offer the possibility of separately recovering both the fibers and the resins. These methods involve the use of a reactive solvent to break polymer bonds and lower temperatures compared to pyrolysis. As a result, these chemical methods have the potential to ensure cleaner fibers that retain more of their strength due to their exposure to lower temperatures [23]. In this process, known as solvolysis, the decomposition of the resin can be achieved by using different combinations of acidic solvents or alcohols, either close to their critical temperature or above it [61,62,63,64]. This method can recycle a wide range of glass and carbon fiber resin composites with high efficiency (almost 100%) [65,66]. Since it recovers clean and intact fibers, as well as the resin, it could lead to a closed cycle for fiber-reinforced resin composites. However, this type of processing presents some operational disadvantages, such as the handling of mixed waste and long reaction times due to thick and dense samples [53]. It also shows some economical disadvantages, such as the cost of energy, catalysts use, and solvent consumption, among others, which end up affecting the competitiveness of the recovered fibers and resin compared to virgin materials [64]. Solvolysis can also be applied for matrix upcycling, representing a highly attractive alternative due to the reduced energy requirements related to avoiding solvent evaporation. For example, Manarin et al. [65] studied the solvolysis of epoxy–anhydride matrices of CFRPs, using different diols under mild conditions. The CFs were recovered, with full retention of mechanical properties, and the polyol resulting from the solvolysis was employed to obtain polyurethane coatings. The same strategy, though with different catalysts and reactive solvents, was applied to epoxy matrices by Zhao et al. [67] and by Liu et al. [68], among others, showing promising results.

Thermal recycling techniques, such as pyrolysis and fluidized bed processing, have shown promise in breaking down the polymer matrix while recovering the valuable glass fibers. However, these methods are still under development and face challenges related to cost and scalability [8,44,50,69,70]. Despite the numerous advantages offered by the above-mentioned technologies, the pursuit of more economically viable alternatives remains ongoing. Currently, the repair of operational wind turbine blades is predominantly manual, involving the application of patches to damaged areas (a labor-intensive process that could be optimized through the introduction of advanced materials and improved replacement strategies) [66]. In this context, the development of novel materials aimed at extending the service life of wind turbine blades and enabling efficient recycling using accessible technologies is a primary focus of current research. Over the past decades, considerable attention has been directed toward the creation of extrinsic self-healing thermosetting polymers. These materials typically incorporate healing agents encapsulated within microcapsules or microvascular networks [71,72,73]. Upon crack propagation, these capsules or channels rupture, releasing the healing agent, which interacts with a catalyst dispersed in the matrix to bond the fractured surfaces. Whilst the service life of the materials can be extended by the autonomous self-healing, a number of disadvantages in this approach prevent it from reaching widespread application: (i) the number of repairs is limited by the amount of the healing agent; (ii) the scar formed in the damaged part is made of a different material, essentially with different properties; (iii) the long-term stability of the healing agent is a key issue, insufficiently addressed so far; (iv) the processing has to be made taking special care not to damage the microcapsules or microchannels, posing significant difficulty in the context of composite manufacturing; (v) the materials retain their non-recyclable nature; thus, even when they reduce the amount of waste, they ultimately only delay the problem of the final disposition [74,75].

A different and more auspicious strategy to overcome the shortcomings of extrinsic self-healing thermosets emerged with the development of Covalent Adaptable Networks (CANs) [75,76,77,78], a novel class of intrinsic self-healing materials that quickly gained attention from both academic and industrial partners. The idea underlying these polymeric networks is that a fraction of the covalent bonds that build up the network can be either reversible (as in the case of dissociative CANs) or dynamic (as in associative CANs) and must be activated by an external stimulus (in most of the cases, heat, but also light, pH, or mechanical stress have been studied, among others) [75,79]. Figure 10 shows schematically the mechanisms for the bond exchanges in both associative and dissociative CANs.

The presence of reversible bonds in dissociative CANs can be harnessed to depolymerize the matrix upon exposure to an external stimulus, yielding monomers or low-weight soluble oligomers in the form of a viscous liquid [80,81]. Dissociative CANs offer the possibility of reusing the polymeric precursors and, within certain temperature ranges, they can be repaired without complete depolymerization, preserving the crosslinking structure to some extent (though with a temporal and local decrease in the crosslinking density) [80]. Associative CANs, also termed “vitrimers” and first introduced by Leibler’s group [82,83], can be repaired and recycled, but the main difference is that their crosslinking density remains essentially constant throughout the thermal treatment. These materials make use of dynamic bonds that can undergo exchange reactions, proceeding through an associative mechanism, in which the new bond is formed at the same time that the original one is broken.

Vitrimers, as well as dissociative CANs, have successfully been applied in the manufacturing of glass and carbon fiber composites. Several studies have shown that these matrices can be healed when incipient, minor damage is generated (Figure 11). Though repair is no longer possible once delamination has occurred, or when the fibers break, many of these systems still offer the possibility of separating and recovering the fibers, and eventually also the matrix components. The early work of Chabert et al. [84] shows a GF composite with a commercial epoxy–anhydride matrix with a Zn²⁺ catalyst to accelerate the transesterification exchange reactions. The composite was able to weld autonomously upon application of heat. Later, Wu et al. [24] obtained a recyclable glass fiber–epoxy composite by resin transfer molding and showed that the fibers could be recovered by dissolving the matrix in a hydrogen peroxide/acetic acid solution. GF composites can also be produced with a matrix synthesized from partially biobased precursors, with high Tg, good mechanical resistance, and the ability to heal scratches (30 min at 230 °C), be recycled (30 min at 220 °C), and be degraded in a KOH/ethanolamine solution [85]. Siloxane-based vitrimers have also been successfully applied to glass fiber composites proposed for wind-energy applications [86,87], showing fast (40 min) and highly efficient (ca. 90%) heat-activated healing.

Vitrimeric matrices were also used for higher performance composites with CFs, and their repairability was assessed, with epoxy vitrimers ranking as one of the most popular ones, thanks to their well-known properties but also to the versatility and availability of a multitude of commercial epoxide and acidic precursors. This enables obtaining vitrimers with a wide range of thermal properties, retaining many of their known advantages. The utilization of epoxy matrices for CF composites is well known, achieving unmatched specific mechanical properties [89,90,91]. Nowadays, a significant part of the research conducted is aimed to the improvement of its repairability and recyclability [92,93,94]. In an interesting approach, a trifunctional epoxy resin was crosslinked with diamine curing agents, obtaining a vitrimer with the covalently bonded tertiary amino groups serving as an internal catalyst for the exchange reactions [95]. These systems are often called “autocatalytic”, and seek to eliminate or reduce the catalyst leaching or its inactivation, that could occur on external unbonded catalysts, especially considering the extended expected service life of the wind blades, during which they are exposed to severe conditions [96,97]. The vitrimer designed by Cai et al. [95] was used as a matrix for carbon fiber composites, showing acceptable thermal and mechanical properties and a very good healing efficiency. Furthermore, the authors showed that the matrix can be degraded in dimethyl formamide at 140 °C thanks to the formation of intramolecular loops, allowing the recovery of the fibers and the upcycling of the polymeric components to produce a polyurethane by reaction with isocyanates. The processability of the new polymers in order to obtain suitable composite parts has also been addressed, such as resin transfer molding (RTM) [98]. Epoxy vitrimer/carbon fiber composites are promising materials, and their applications in the field of energy are not limited to the construction of wind blades. For example, in the energy storage area, linerless pressurized vessels can be used for the transportation of hydrogen, where they are subjected to fatigue-induced microcracking [89,95,99]. The repair of the microcracks generated in the vessels not only helps prevent catastrophic failure, but also restores their functional properties, keeping the pressurized gas from leaking out [99]. In a recent review, Fan et al. summarize the main chemistries used in recyclable CFRPs [100].

The application of CANs in the manufacturing of CF composites is rapidly gaining momentum as the consumers push for more sustainable alternatives. “Recyclamine”, a recyclable curing agent developed by Connora Technologies [101,102], was also applied as a matrix for CF composites, allowing for the recovery of the fibers through mild solvolysis (1 h at 80 °C in a 25% aqueous solution of acetic acid). Part of the matrix was also recovered as a thermoplastic material. Mallinda, a US based company, produces vitrimeric materials based on imine exchangeable bonds, designed to be used in composites with a high recyclability through solvolysis [27,103]. A clear disadvantage of imine dynamic bonds, which still has to be tackled, is that they undergo a severe degradation of their mechanical properties in the presence of water or in high-humidity environments [104]. The utilization of recyclable crosslinked polymers has already extended to the fabrication of wind turbine blades: the first specimens of a new generation of recyclable blades developed by Siemens Gamesa under the trademark “RecyclableBlade” have already been installed in recent years [23]. These are a new type of wind turbine blade, made from an innovative resin system that allows for easier material recovery at the end of the blade’s life. This development represents a critical step toward minimizing the environmental footprint of wind energy systems and ensuring that they align with circular economy principles. Wood has also been evaluated as a sustainable alternative for wind blades [105]. The Swedish company Modvion is also developing wooden wind blades, and Voodin Blade Technology installed the first wooden wind blades in 2024 in Germany [106]; however, large blades made of wood are still in an experimental phase.

The technological improvements mentioned so far are undoubtedly a necessary step toward increasing the sustainability of currently available energy harvesting systems, providing a feasible path to the reutilization of the materials, including both the fibers and the matrices. Nevertheless, many issues regarding the in situ repair of the matrices still remain a challenge. The activation of the dynamic bonds requires an external stimulus that has to be delivered, ideally only to the damaged area, and significant research efforts are being devoted to overcoming this limitation. The addition of nanostructures (mainly metallic nanoparticles or carbon nanotubes) to the polymeric matrix is a very promising approach, allowing for the remote activation of the exchange reactions through irradiation with a proper wavelength [88,107,108,109,110,111,112]. In this case, a light source with enough power should be used to induce heating by focusing it on the damaged parts, preventing the disassembling of the device. The inherent absorption ability of CFs in the near IR region can also be harnessed for this purpose, simplifying the production of composites and avoiding the need for additional reagents and production steps [99,113].

Damage reporting is another highly useful feature, since the healing of small cracks at an early stage is in all cases much more efficient than massive failure (which is actually, in most cases, impossible to heal through these methods), hence the importance of detecting the cracks as soon as they are generated. The addition of mechanophores to the polymeric network has been proposed as a promising and reliable tool to timely detect the damages [91,114]. Duchene et al. [115] reviewed state-of-the-art non-destructive methods to assess damages in composites, but most of the techniques cannot be applied in situ, and usually more than one is required for a reliable evaluation. Up to now, and to the best of our knowledge, there are still no reversible and efficient mechanisms for the self-detection of cracks. It should be made clear that, whilst being appealing, these technologies are still in their infancy, and further work is needed before they can be safely applied for wind blades and other high-demanding uses.

In conclusion, while wind turbine blades offer significant environmental benefits through clean energy generation, their end-of-life management remains a critical challenge. Figure 12 shows a summary of the main current challenges and the solutions proposed so far. Innovations in blade design, combined with advances in recycling technologies, are essential for addressing the growing volume of waste and ensuring that wind energy systems contribute to a sustainable future.

3.3. Transmission

The long-distance transmission of electricity is a critical component of the renewable energy infrastructure, especially with the expansion of high-voltage direct current and high-voltage alternating current systems. These systems require highly efficient insulation materials to ensure performance and safety, with crosslinked polyethylene being the preferred material for high-voltage cable insulation. XLPE differs from its linear and branched homologues in that its crosslinked structure (Figure 13) provides it with a higher thermal stability, which is necessary to withstand the harsh conditions that cables are exposed to.

3.3.1. Current Use of XLPE in Transmission Cables

XLPE has become the dominant material for cable insulation in HVDC and HVAC systems due to its excellent dielectric properties, mechanical strength, and thermal stability [25]. Crosslinking polyethylene transforms it into a material capable of withstanding the electrical, thermal, and mechanical stresses encountered in long-distance energy transmission. This ability to maintain dimensional stability at high temperatures makes it ideal for the insulation of high-voltage cables. Furthermore, XLPE insulation exhibits low dielectric losses, ensuring the efficient transmission of electricity with minimal energy waste [26].

However, despite its advantages, XLPE poses significant sustainability challenges. The crosslinked structure, which gives XLPE its mechanical and thermal resilience, also increases the processing efforts and renders it non-recyclable [15]. After decades of research, conventional XLPE could be reaching the limits of its capabilities, and further development could therefore be subject to diminishing returns [116]. The costs of large extrusion and catenary crosslinking manufacturing facilities and the costs and time of degassing larger cross-section HV and extra-high-voltage cables present significant sustainability issues for cable manufacturers [25]. Furthermore, at the end of its life, XLPE must be disposed of as waste, contributing to the growing environmental impact of energy infrastructure systems. With the expansion of HVDC and HVAC networks, the disposal of XLPE insulation is becoming an increasingly urgent issue, especially as long transmission lines generate considerable amounts of waste. Figure 14 briefly describes the main open challenges in terms of sustainability related to the massive use of XLPE, as well as some prospective approaches to tackle them.

3.3.2. Performance Enhancements and Lifespan Extension

Efforts to extend the lifespan and improve the insulation performance of XLPE have focused on different solutions. One of the most promising ones relies on the incorporation of nanocomposites such as SiO₂, alumina, TiO₂, and MgO [35,117]. These materials significantly improve the electrical properties of polyethylene, including increased resistivity and dielectric strength. Nanoparticles reduce space charge accumulation and improve resistance to partial discharges, electrical trees, and water trees. Finally, the surface modification of these nanoparticles further enhances their ability to reduce electrical conductivity and space charge accumulation [118]. Efforts have also been made to identify defect levels likely to cause the failure of solid dielectric transmission-class cables and to rehabilitate materials to restore their AC breakdown strength to original values. While these innovations extend the useful life of XLPE in transmission applications, they do not solve the recyclability challenge. As the demand for high-voltage cables increases, the need for more sustainable insulation materials that can either be recycled or reused becomes critical.

Another challenge in the production of XLPE-insulated HVDC lines is the need to remove unwanted volatile byproducts generated during the radical crosslinking of XLPE with dicumyl peroxide (DCP), especially the emission of methane, whose effect as a greenhouse gas is stronger than that of carbon dioxide in terms of global warming (in both the short and medium term), as shown in Figure 15A. These byproducts tend to increase electrical conductivity, and this removal process is complex and energy-intensive, in addition to releasing methane into the environment [116].

Müller et al. have proposed the use of byproduct-free crosslinking concepts that mitigate these associated problems, reducing energy costs and methane emissions [119,120]. The click-type crosslinking of polyethylene copolymer blends containing glycidyl methacrylate and acrylic acid comonomers (Figure 15B) is a promising alternative to the currently used XLPE processing, with similar thermal, mechanical, and electrical properties.

3.3.3. Recycling Strategies for XLPE

In addition to developing alternative materials, there are ongoing efforts to improve the recyclability of traditional, already in-use, crosslinked polymers, such as XLPE. These materials, widely used for their excellent thermal, mechanical, and chemical resistance, are difficult to recycle, leading to the accumulation of waste, primarily through landfilling or incineration. Innovative recycling strategies have been proposed to address these issues, focusing on breaking down the crosslinked structures to allow material reprocessing and reuse. Figure 16 provides a summary of the most common strategies.

One promising approach is chemical recycling, which aims to break the crosslinked bonds in polymers, enabling the reprocessing of the material. For example, supercritical water technology has been shown to successfully break the crosslinking bonds in XLPE, allowing it to be reused as a thermoplastic resin. This process involves treating XLPE with supercritical water to cleave the covalent bonds, enabling it to retain properties similar to the original material, thereby facilitating its reuse in new applications [121]. Although effective, the scalability of this method remains a challenge due to the high energy requirements and costs associated with maintaining the supercritical water conditions.

Another notable technique involves thermoplastic recovery technology, which is designed to make crosslinked polymers more amenable to reprocessing. By decreasing the crosslink density, this technology allows crosslinked polymers such as XLPE to be recycled into a form that behaves similarly to thermoplastic materials. The thermoplasticized material can then be processed at elevated temperatures, potentially extending the lifespan of the polymer [122]. This approach offers a more energy-efficient and cost-effective solution, though further optimization is needed to ensure that the recycled material retains the desired mechanical and electrical properties.

Nitroxide-mediated polymerization (NMP) represents another innovative strategy for creating recyclable crosslinked polymer networks. This method involves a one-step polymerization process that allows the formation of crosslinked polymers that can be melted and reprocessed multiple times while maintaining their structural integrity [123]. NMP offers a pathway toward recyclable crosslinked polymers that could significantly reduce waste in industries that rely heavily on such materials, including renewable energy systems. Supercritical water processing and thermoplastic recovery technologies offer promising routes to reduce waste, but the future of crosslinked polymer recycling will likely involve a combination of chemical, mechanical, and thermal approaches to maximize the recovery and reusability of these materials. Despite these advancements, recycling XLPE and other crosslinked polymers remains a formidable challenge due to the irreversible nature of the covalent crosslinking. The vast majority of industrial waste from crosslinked polymers is still either landfilled or incinerated, contributing to environmental degradation. Other methods include technologies for decrosslinking based on ultrasonically assisted single (SSE) and twin screw (TSE) extruders [124].

Continued research into these methods is crucial to addressing the environmental impact of crosslinked polymers, particularly in industries such as renewable energy, where sustainability is of paramount importance.

3.3.4. Innovative Alternatives to the Use of XLPE

To address the sustainability challenges posed by XLPE, researchers have been exploring alternative materials that provide equivalent or superior performance while enabling improved end-of-life management [116,125].

An interesting approach to developing sustainable alternatives involves creating new polyethylene-based materials without the need for chemical crosslinking, which allows the material to be fully recyclable while maintaining the necessary mechanical and electrical properties for high-voltage cable insulation [118,126,127,128,129]. These developments also focus on eliminating the complex and energy-intensive processes involved in XLPE production, offering a path toward more sustainable, circular material systems.

Binary blends of linear (LPE) and branched (BPE) polyethylene have been investigated as an alternative to XLPE. These blends exhibit enhanced mechanical and thermal properties compared to conventional crosslinked polyethylene, showing potential for replacing XLPE and for operating at high temperatures [130]. Although at 130 °C (the XLPE overload temperature) the LPE/BPE blend would melt completely, the authors found that the material could acceptably withstand temperatures up to 120 °C for short times.

Alternative materials such as isotactic polypropylene (i-PP) [37] and propylene-ethylene copolymers [38] are also being explored, offering enhanced mechanical properties at higher temperatures while maintaining strong dielectric performance. A propylene-ethylene copolymer demonstrated enhanced mechanical properties at elevated temperatures while maintaining strong dielectric characteristics. These copolymers not only meet the electrical insulation requirements for high-voltage cables but also show potential for better recyclability compared to XLPE [26]. Their superior high-temperature mechanical integrity makes them particularly suitable for environments where XLPE might begin to fail. Although these materials are not yet widely used, the attention that these materials have drawn from industrial partners has recently led to the filing of a patent [131].

A particularly significant and interesting advancement in this area is the development of polyethylene-based ionomers. These materials are based in high-pressure free-radical copolymerization in the presence of ethylene and a suitable initiator of ion-pair comonomers (Figure 17) and offer thermo-mechanical and dielectric properties similar to those of XLPE while being fully melt-processable and recyclable [39].

Unlike XLPE, which cannot be reprocessed, ionomers can be reused at the end of their service life, providing a sustainable solution for high-voltage cable insulation. Ionomers are synthesized without the harmful byproducts typically generated during the chemical crosslinking process, further reducing their environmental footprint [39]. The recyclability of ionomers, combined with their high performance, makes them a highly attractive alternative to traditional crosslinked polymers in renewable energy transmission systems. In a short review, Jia et al. [132] highlight the potential of ionomers as HV insulators, and mention that research in this direction has already been conducted for three decades [133].

3.4. Other Applications

Our analysis also shows other applications in which crosslinked polymers can play a minor role, as in the case of energy storage, especially in electric vehicle batteries. With the global shift towards electric vehicles (EVs) as a critical strategy for reducing greenhouse gas emissions, the demand for efficient and sustainable energy storage systems is rapidly increasing. At the heart of this transition are automotive batteries, particularly lithium-ion batteries (LIBs), which rely on complex material systems to ensure safety, performance, and durability throughout their lifecycle. Crosslinked polymers are among these materials, particularly in separators and electrolytes, where they provide mechanical stability, thermal resistance, and ionic conductivity [134].

Regarding structural integrity and thermal stability in modern electric vehicle batteries, the crosslinked polymer membranes play a critical function. These materials, used as separators and gel-polymer electrolytes, offer superior thermal and electrochemical stability compared to traditional polyolefin separators, which are limited by their lower thermal resistance and mechanical properties [135]. For example, polyimide-based separators have shown particular promise due to their high-temperature resistance and mechanical strength. Known for their exceptional thermal stability, polyimides can withstand temperatures up to 300 °C without significant degradation. This makes them highly suitable for LIBs, where heat management and mechanical resilience are critical for ensuring safety and performance. These polyimide separators are currently being explored for their ability to improve battery safety and potential for high-energy-density applications [136]. To further enhance the properties of crosslinked polymer membranes, several preparation techniques have been developed. Thermal crosslinking and alkaline hydrolysis are commonly used methods that improve the mechanical integrity and chemical stability of the membranes. Crosslinked fiber membranes, in particular, have demonstrated excellent thermal and electrochemical properties, making them suitable for use in LIBs, where both ionic conductivity and mechanical strength are required for optimal cycling performance [137]. In particular, the development of crosslinked fiber porous membranes from materials such as polyacrylonitrile (PAN) has enabled enhanced electrolyte retention and improved ionic conductivity, which are crucial for high-performance applications.

On the other hand, in the field of gel-polymer electrolytes (GPEs) in situ crosslinking has emerged as a promising technique for improving thermal stability and ionic conductivity. Crosslinked gel-polymer electrolyte membranes have demonstrated excellent performance, with some formulations being able to retain their structure and ionic conductivity at temperatures as high as 260 °C [138]. These membranes exhibit not only higher ionic conductivity but also an improved cycling performance, making them ideal for the development of solid-state batteries. Solid-state batteries are widely considered the next step in lithium-ion technology, due to their higher energy density and improved safety compared to traditional liquid electrolyte systems. Moreover, novel crosslinked polymer formulations, such as polyether-SiO nanocomposites, have been investigated for their combined mechanical stability and electrochemical performance. These hybrid materials exhibit enhanced electrochemical stability and thermal resistance, further supporting their use in high-performance applications where traditional materials may fall short [139].

Nevertheless, the use of crosslinked polymers in battery components is not just about improving performance but also about enhancing safety. LIBs are prone to thermal runaway, a condition where excessive heat generation can lead to dangerous consequences, including fires or explosions. The thermal stability of crosslinked polymer membranes mitigates these risks. For instance, the above-mentioned polyimide-based separators and crosslinked gel-polymer electrolytes ensure that the battery structure remains intact, even under high temperatures, significantly reducing the risk of short circuits or thermal runaway [136,138]. Additionally, crosslinked polymers help extend battery lifespan by maintaining mechanical integrity during repeated charge–discharge cycles.

3.4.1. End-of-Life Challenges and Recycling of Battery Components

The end-of-life management of LIBs presents significant environmental and economic challenges as the adoption of electric vehicles continues to accelerate [140]. The increasing volume of spent batteries is expected to result in over 11 million tons of waste by 2030, demanding efficient recycling strategies to manage this growing waste stream. Current recycling efforts primarily focus on the recovery of valuable metals, such as lithium, cobalt, and nickel, using hydrometallurgical and pyrometallurgical processes [141]. However, these techniques often neglect the polymeric components, which are either incinerated or sent to landfills, contributing to environmental pollution and the waste of valuable materials [142].

The complexity of recycling LIBs stems from the diverse materials used in their construction. Crosslinked components are especially difficult to recycle due to their covalent network structure, which render them non-reprocessable through traditional recycling methods [143]. However, the need to reduce the environmental footprint of LIBs has driven researchers to explore sustainable alternatives for battery materials, including recyclable crosslinked polymers, upcycling strategies, and the use of natural feedstocks [141]. As in the case of wind turbine blades, one promising avenue for improving the recyclability of battery components lies in the development of reprocessable crosslinked polymers, such as vitrimers and ionomers. However, thermoplastic elastomers are also gaining attention as potential replacements for crosslinked polymer separators. These materials offer similar thermal and mechanical properties as crosslinked polymers but with the added benefit of being recyclable. Their reprocessable nature allows for the recovery and reuse of polymeric components from spent batteries, thereby reducing the overall environmental impact [143].

3.4.2. Regulatory and Economic Considerations

As mentioned before, recycling processes tend to focus on recovering high-value metals such as cobalt, which offer more significant economic returns due to their market value and role in battery production [144]. However, as the industry moves towards cobalt-deficient and mixed-metal cathodes, the economic incentives for recovering cobalt are expected to decline. This shift will necessitate the development of more holistic recycling strategies that emphasize the recovery of other essential materials, such as lithium, nickel, and manganese, which are crucial for maintaining the supply chain of battery production [145].

The regulatory landscape also plays a crucial role in shaping the future of LIB recycling. The European Union (EU) has taken significant steps by introducing regulations for the recycling of spent LIBs. These regulations aim to ensure the recovery of critical materials while minimizing environmental harm [146]. For instance, tax incentives and government policies can make LIB recycling more economically viable by encouraging the establishment of recycling facilities and reducing operational costs [147]. Such policies could serve as a model for other regions, promoting the adoption of circular economy principles and ensuring that spent batteries are properly recycled rather than sent to landfills.

As the market for electric vehicles grows, so does the urgency of addressing logistical challenges in LIB recycling. Optimizing the collection and reverse logistics of spent batteries is essential for maximizing material recovery and ensuring that critical resources such as lithium are efficiently recycled [148]. In addition to recycling, reuse strategies for LIBs, such as second-life applications in less demanding environments, could further reduce the environmental impact and make recycling efforts more economically viable. Future regulations may increasingly focus on end-of-life responsibilities for manufacturers, encouraging them to design batteries with easier recyclability in mind [149]. Collaborative efforts between manufacturers, recyclers, and government bodies will be necessary to develop and implement recycling processes that are not only environmentally sustainable but also economically feasible.

4. Conclusions

This review has critically analyzed the sustainability challenges posed by crosslinked polymers used in renewable energy systems, with a specific focus on their role in production and transmission. An initial assessment of the current literature, based on the analysis of a complex citation network, showed that a significant amount of academic research is currently being directed to the development of new polymer-based materials for the production and transmission sectors and, to a lesser extent, to the storage of electric power. The citation network showed that the non-recyclable nature of polymers used nowadays in these fields remains a major environmental concern, as it contributes to significant waste accumulation at the end of their lifecycle. While alternative materials such as covalent adaptable networks, ionomers, or thermoplastic copolymers show promise in improving recyclability and reducing waste, substantial challenges still need to be overcome.

In the context of energy production, the end-of-life management of wind turbine blades poses a critical challenge. Innovations such as “RecyclableBlades” represent crucial steps forward, but a broader adoption of circular economy principles is required. Moreover, other innovations would be needed in order to successfully and efficiently repair damaged wind blades made from recyclable or healable polymeric matrices. This would represent a first step toward the reutilization of their constituent materials, enhancing the environmental benefits of the applications of recently developed polymers. Similarly, in the energy transmission sector, the dominance of XLPE in high-voltage cables has led to a growing environmental burden. Though nanocomposite modifications and new copolymers have extended the material’s operational life, a complete transition to reprocessable materials such as ionomers is necessary for long-term sustainability. Finally, in the area of energy storage, the use of crosslinked polymers in lithium-ion batteries complicates recycling efforts, with current processes largely ignoring polymeric components. The development of vitrimers, ionomers, and self-healing polymers for use in battery separators and electrolytes provides a path forward, allowing for improved recyclability and extended battery life. Future research must focus on integrating these materials into commercial applications and refining recycling technologies to handle the growing volume of spent lithium-ion batteries.

From a wider perspective, other areas beyond the scope of this review are also seeing important advances aimed at mitigating the footprint of human activities. For instance, regarding solar energy harvesting, developments such as reprocessable encapsulants in solar panels can also provide valuable support in meeting the principles of circular economy.

The development of composite recycling techniques and new recyclable polymer materials in the field of renewable energy is driven by initiatives such as the UN Sustainable Development Goals and the EU Extended Producer Responsibility (EPR), which incentivize the adoption of these new technologies to increase the sustainability of energy generation and transmission. Above all, EPR seeks the use of environmentally friendly materials, requiring producers to assume the costs of waste management. This encourages the design of materials with reuse or recycling techniques in mind, and their adoption to reduce management costs due to their easy reintegration into the production cycle. These are actions that favor the transition toward a circular, low-carbon economy.

Ultimately, the transition to more sustainable and recyclable polymer technologies is essential for the future of renewable energy systems. Continued research and innovation in this field will help to reduce the ecological footprint of renewable energy infrastructures and contribute to the global pursuit of sustainability.