## 1 Introduction  
Nowadays, lithium-ion batteries (LIBs) are the most consumed energy storage and supply devices for portable electronics. Their characteristics (high energy density, long cycle life, high roundtrip efficiency, wide range of operating temperature, high reliability, safety, fast recharge and low self-discharge rate) make them the best option for numerous applications. LIBs play a vital role in the development of electric vehicles and in the storage of energy from renewable sources [1, 2], which will contribute to decreasing the global carbon emissions. The increasing demand for LIBs from the sectors of portable electronics and transportation comes together with an increasing interest for recycling of spent batteries [2, 3, 4]. The main limitations for the implementation of proper recycling procedures are the lack of standards for the physical formats and the frequent appearance of new batteries with different chemistries [1]. There are normally three different shapes: prismatic, pouch (or polymer) and cylindrical cells. With respect to the chemical composition, LIBs consist of a cathode, an anode, an organic electrolyte and a separator. In most of the LIBs currently commercialized, the anode is made of graphite cast over copper foil. Nowadays, alternative anodes are under study, such as Li\({}_{4}\)Ti\({}_{5}\)O\({}_{12}\), graphite nanotubes or nanoparticles, or compounds of Si and Sn. The cathode, supported on an aluminum foil, is usually built from transition metal oxides, such as LiMn\({}_{2}\)O\({}_{4}\) (LCO), LiFePO\({}_{4}\) (LFP),LiMn\({}_{2}\)O\({}_{4}\) (LMO), Li(NiMnCo)O\({}_{2}\) (NMC) and (NiCoAl)O\({}_{2}\) (NCA) [5]. Most of the toxic and valuable metals are found in the cathode, which represent approximately 25-30% of the total battery mass. According to the European Union, some components of LIBs, such as cobalt, phosphorous and natural graphite, are classified as \"Critical Raw Materials\" (CRMs) due to their high supply risk and economic importance [6]. Hence, recycling technologies for any CRM-containing waste are in the spotlight of the EU development plan. Nowadays, most battery recycling technologies focus on the recovery of Co, as it is considered the bottleneck in the battery industry, leaving the recuperation of Li in the background [1]. However, the increase in the LIBs market may place Li on the list of critical materials by 2030 [1,7,8]. Indeed, the increasing demand of Li for batteries may soon produce a supply shortage, which promotes the investigation of Li-free alternatives such as sodium or aluminum ion batteries [9]. In addition to the supply chain risk, some of the materials used in LIBs represent a threat to the environment, for example, the emissions of fluoride gas from battery fires or the aqueous contamination from the metals in the cathode. This environmental threat also motivates the development of recycling initiatives [10,11]. Regarding the currently existing LIBs recycling technologies, they can be classified into pyrometallurgical, hydrometallurgical, biometallurgical and combined techniques. Pyrometallurgical processes use high temperature smelting to recover cobalt, copper and nickel alloys [2]. Hydrometallurgical processes use chemical acidic leaching to dissolve the metal containing components, followed by chemical separation and recovery [12]. In biometallurgical processes, microbial activity promotes the production of inorganic and organic acids to leach metals from spent LIBs [13]. The main limitation of these techniques is that lithium is lost in slag, and they have high energy and chemical reagents consumption [3,14]. Up to 50 companies around the world recycle lithium-ion battery at some scale, most of them located in China, South Korea, Europe and North America [4]. Some important companies, such as Umicore, Duesenfeld, Toxco and Recupyl have developed industrial-scale recycling processes applying the aforementioned techniques [15,16]. As an innovative recycling process, [PERSON] et al.