Metal-ion batteries (MIBs), particularly lithium- and sodium-ion ones (LIBs and SIBs, respectively), are an essential part of portable and stationary electronics due to their energy density characteristics and long-term cycling ability. The expected growth of the MIBs market unavoidably leads to the generation of tons of spent batteries, this in turn dictates the necessity of a proper life cycle management of used cells and packs.
The development of energy harvesting and storage technologies is an integral part of changes under way in many sectors of the economy and everyday life, spanning from portable electronics and electric vehicles to local power supplies and renewable electricity generation 
. Currently, lithium-ion batteries (LIBs) are one of the key enablers for efficient energy management. However, the rapidly growing LIB production faces a number of issues, such as unevenly distributed Li resources, long and intermittent supply chains, negative socio-economic aspects of raw materials mining, and environmental concerns 
. This in turn has stimulated research on complementary metal-ion battery (MIB) architectures, with notable progress achieved in recent decades 
. Among them, sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) are considered as reasonable candidates for several niche applications, for example, at large-scale stationary storage facilities 
. The expected continued growth of the MIB market will unavoidably bring about the generation of tons of spent batteries demanding proper recycling and utilization. The annual growing large-scale production of batteries raises concerns about the disposal of its significant number that have lost ability to perform their functions (worked out the declared number of cycles, with a manufacturing defect, of expired service). When released into the environment, batteries can cause irreparable environmental damage and threaten the safety of ecosystems. These problems can be solved through realization of a closed-loop cycle economy that requires the development of regulatory frameworks and specific technologies. This statement is strongly supported by recent decisions made by several countries, as covered in the following reviews 
The global research community is taking a number of steps to develop technologies for handling hazardous battery waste. The greatest attention is paid to the preparation of raw materials for recycling from spent cathodes and anodes, which account for at least 40% of the battery mass. To date, recycling methods have been developed for processing the most widely used electrode materials based on compounds of cobalt, nickel, manganese, aluminum, and lithium.
2. Overview of Metal-Ion Battery Recycling Methods
Inevitable performance degradation during operation limits the lifetime of any MIB, thus determining the necessity of developing recycling methods for used batteries. The degradation processes can be divided into two groups: physical and chemical. Chemical degradation is driven by irreversible structural transformations, which lead to a capacity loss, and/or by partial dissolution of metal cations and various reactions that occur on the electrodes’ surface coupled with electrolyte decomposition 
. Physical (or mechanical) degradation is associated with defect formation and stress accumulation, resulting in a particles cracking and detaching, which mainly occur due to the crystal lattice breathing as a result of mobile ion insertion and removal during cycling 
. In many cases (overcharge, physical damage, etc.), the battery cannot be reused; thus, full disassembly with subsequent recycling should be applied. The initial stage of recycling processing usually includes careful sorting, pre-discharge and, if necessary, partially or fully automated disassembly 
. The classification of battery recycling methods will follow and is schematically shown in Figure 1
Figure 1. Schemes of direct (green), pyrometallurgical (orange), and hydrometallurgical (light blue) recycling methods for recovery of active materials in MIBs. Violet depicts initial electrode material synthesis from raw resources, battery assembly and use. TM = transition metal, AM = alkali metal.
(1) Pyrometallurgical methods were first widely implemented in the industry. The approach implies multi-stage heating. At low temperatures (<200 °C), the electrolyte evaporates, and the polymer binder melts. Further heating at higher temperatures is necessary to burn the battery case and other component polymer-containing fractions. The required temperature, as a rule, depends on the specific technology and composition of the batteries being processed, but can reach 1500 °C. The products of the pyrometallurgical process are metal alloy, slag and gases. The resulting flux is leached to obtain pure metal compounds, and the slag is separated. Normally, the slag contains a certain amount of lithium, but a significant proportion of it is lost with vapors and evolved gases. The search for methods to reduce lithium losses in the pyrolysis process is a separate technological problem in the field of metallurgy and chemical technology. The disadvantages of this approach include the low quality of the extracted materials, high energy costs, as well as high capital expenditures due to the need to capture, clean up and utilize harmful gases 
(2) Hydrometallurgical approaches are based on reactions in solutions. Dissolution of electrode materials with a subsequent extraction in the form of pure salts, oxides or hydroxides are key technological units this group of methods 
. Materials previously separated from other parts of the battery are most often leached by acidic aqueous solutions, with sulfuric acid and hydrogen peroxide being the most common combination of inorganic reagents used 
. Also of practical interest are organic acids, which are large-scale products of the chemical industry and whose anions can act as complexing agents (for example, oxalic, citric, malic, tartaric, succinic, ascorbic acids) 
. At the next stage, metal salts are isolated either through a selective precipitation by adjusting the pH value of the solution, or by extraction using organic solvents. The unconditional advantages of hydrometallurgy include the conversion of metal compounds into a soluble form convenient for further use, as well as lower temperatures required for the processing procedure, which reduces its energy consumption. Hydrometallurgical production is characterized by a higher degree of extraction of valueable compounds if proper processing and sorting of raw materials have been performed. However, it requires using high-concentration leaching agents and leads to the formation of large amounts of waste solutions; all of these features increase the number of technological units.
(3) Direct recycling consists in the regeneration of the used electrode (usually cathode) material without introducing a significant deviation from its chemical composition. The used MIBs are sorted and further disassembled into individual components. At the next stage of production, the electrode mass is separated from the aluminum tape and calcined to restore the crystal structure of materials suitable for use in storage devices. Heat treatment allows one to decrease the number of defects in the crystals and to remove organic residues that form during prolonged cycling and negatively affect the electrochemical performance. This method is the most advantageous in terms of lithium recovery efficiency. The main drawbacks of this approach are the technical complexity and complicated time-consuming processes, which increase production costs. In addition, this group of methods can also include the steps of disassembling the used batteries, mechanically cleaning possible foreign inclusions, and filling with a fresh portion of an electrolyte 
The vast majority of works related to the processing of MIB materials are aimed at studying the process of leaching and selective extraction of Li, Co, Ni and Mn compounds. Typical objects of study are widely used layered oxides (mainly Li-NMC, Li-NCA) 
, oxides with a spinel structure (LiMn2
, as well as triphylite-structured phosphates (LiFePO4
. To date, the enterprises of industrial groups have implemented a joint approach for the processing of batteries, combining pyrometallurgical and hydrometallurgical methods. The application of pyrometallurgical methods is primarily due to the lack of proper labeling system in batteries industry, as well as significant differences of sizes and form factors of battery packs. A small proportion (a few percent) of used batteries are subjected to “direct recycling” (or regeneration), implying that they are manually disassembled in an inert environment, filled with a fresh portion of the electrolyte, and subsequently sealed 
. The latest developments in the direct recycling methods are more of a technical problem at the intersection of chemistry, chemical technology, and engineering; their investigation is of particular interest for some niche applications.