The concerns of oil depletion, increasing oil prices, rapid climate change, and greenhouse gas emissions have spurred leading economic countries to increase their search for alternatives to conventional energy. Electric vehicles (EVs) powered by batteries are a promising technology that are already in commercial use.
But where do batteries go when they die? In this article, we’ll take a look at the benefits, challenges, and technologies of battery recycling.
Battery Recycling
Batteries which have reached the end of their service lives should be recycled. Batteries contain extremely hazardous components such as mercury, lead, cadmium, nickel, zinc and cobalt, all of which have a considerable negative effect on the environment. Careless disposal can lead to leakage of these hazardous substances and heavy metals, endangering the health of populations and contributing to the pollution. The battery housings corrode, which can lead to chemical contamination of the soil and water, and some battery types can cause fires under certain conditions.
The increased usage of EVs has exacerbated the need for proper battery recycling. Fortunately, the problem has been recognized and the number of battery recycling processes in the world is increasing.
Since the specific materials used in EV batteries are available only in select countries, access to resources is crucial for ensuring a stable chain supply. In the future, EV batteries may prove to be a valuable secondary resource for critical materials. It is argued that cobalt-based batteries should be recycled immediately to improve cobalt supplies.
The most commonly used battery type in EV applications is lithium ion (Li-ion). However, due to the widespread use of Li-ion batteries, there could be a shortage of raw materials in the future, as well as serious environmental consequences unless the batteries are properly recycled. The benefits of battery recycling are both economic and environmental. Battery recycling preserves the limited raw materials and protects the environment by reducing landfill waste.
Pre-recycling Processes
Storing aged batteries is potentially unsafe (and unsustainable), so if a battery cannot be reused, then it should be repaired or recycled.
Most battery recycling processes require a disassembled battery, at least to its module level. However, battery dismantling is not a simple process and there are numerous associated hazards. High voltage training and insulated tools are required to prevent electric shocks to personnel or short circuits when disassembling the batteries from electric vehicles. Short circuits result in rapid discharge, which can lead to the so-called battery “thermal runaway.” Ultimately, this can release particularly hazardous by-products, including hydrogen fluoride (HF) gas, which, along with other gases, can accumulate and eventually result in an explosion of the cell. Battery cells contain flammable electrolytes, toxic and carcinogenic electrolyte additives, and potentially toxic or carcinogenic electrodes, which all present a chemical risk.
Battery Dismantling Challenges
Battery structures further complicate the recycling process. Li-ion batteries are compact, complex devices, delivered in various sizes and shapes, and they are not designed to be disassembled. In simple terms, each cell contains a cathode, anode, separator, and electrolyte. The components are tightly wound or folded and safely packaged in a plastic or aluminum case. Different EV manufacturers use different approaches for supplying the EVs. Also, EVs available on the market have a wide variety of different physical battery configurations, cell types and chemical components. The multitude of forms represents one of the bigger challenges for battery recycling.
There are three commonly used types of battery cells: cylindrical, prismatic and pouch cell (illustrated in Figure 1).
Figure 1. Comparison of Li-Ion battery cell forms. From left to right: cylindrical, prismatic, and pouch. (Source: JMBS [1].)
The Tesla Models S and X use cylindrical battery cells manufactured at Tesla’s Gigafactory with Panasonic. The Tesla Model S uses 16 modules per pack and 444 cells per module. However, Tesla is considering using prismatic Li-ion-phosphate (LFP) battery cells for cars manufactured in China.
BMW’s EVs use prismatic battery cells (Figure 2). The BMW i3 has 8 modules per battery pack and 12 cells per module.
The Nissan Leaf has an integrated pouch battery cell type with 48 modules per pack and 4 cells per module.
Figure 2. The battery system in BMW i3. Nominal voltage: 360V, Capacity: 40.0kWh, Weight: 278kg, Dimensions: 1660 x 964 x 174mm. (Source: Torqeedo.)
All three package forms have very different physical configurations, which require different disassembly approaches, especially with regard to automation. Prismatic cells have flat (film-like) electrodes. Cylindrical cells are tightly wound. It is essential to keep in mind that manufacturers also use different chemical components, which require different recycling approaches that have a strong impact on the overall economic viability.
Battery Diagnostics
Reusing a battery in other applications (such as the charging stations and stationary energy storage) requires a precise assessment of battery health for categorizing whether the batteries are suitable for reuse (and if so, for which application).
State of Health (SOH) is a measurement of the general condition of the battery in comparison to a new battery. Factors such as charging performance, battery internal resistance, terminal voltage, and self-discharge rate are taken into account. The SOH gives information about the battery’s long-term capacity and provides an indication of how much a battery’s lifetime energy throughput has been consumed and how much is left. The battery energy throughput represents the total amount of battery energy that can be stored and delivered over its lifetime. This information could be noted in the battery warranty certificate. The SOH estimates how much time is left before the battery needs to be replaced, helping supervisors anticipate the potential problems and plan a replacement strategy.
State of Charge (SOC) is a rate of the battery charge or discharge provided in percentage (0% is empty and 100% is fully charged). The SOC benchmark is often defined as the current cell capacity.
Electrochemical impedance spectroscopy (EIS) can provide information on the so-called battery health, as well as indications of aging mechanisms such as lithium coating. EIS can be used as a basis for deciding whether to reuse or recycle a battery, and more importantly, for identifying potential hazards that can have significant consequences for further processing. EIS is also used for testing the batteries in primary production. EV manufacturers plan to use similar technologies for operating and maintaining the batteries for EVs and replacing defective modules in the field.
More information about battery state estimations can be found in Battery Management Systems – Part 2: Battery State.
Preparation Processes for Battery Recycling
Once a battery is designated for recycling, it undergoes several processes: battery passivation, unsealing and dismantling of the cells (material separation), and shredding and breaching of the cells.
Batteries lose performance mostly because of the charge/discharge cycles which cause solid formations in the battery cells. These unstable components are prone to decompose at temperatures higher than 194 °F (90 °C), which releases flammable gases and oxygen. These components create a passivating film on the anode surface, limiting electrochemical reaction and increasing internal battery resistance. Passivization—removing the passivating layer—is important for making flammable chemistries safe from fire hazards.
Passivization can be achieved when the battery is connected to the load. The passivating film’s high resistance causes a rapid voltage drop. The battery discharge process slowly removes the film, decreasing the internal resistance. The voltage recovers to the steady stabilized value if the load remains constant. By increasing the external load, the voltage will drop again and the process will continue until the passivation layer is completely removed.
After passivization and dismantling, the battery components are shredded with a shredder or high-speed hammer. The material is submerged in caustic water (sodium hydroxide, NaOH) which neutralizes the electrolytes, thus recovering ferrous and non-ferrous metals.
Battery discharging is an important consideration prior to shredding, as it has important safety implications. However, discharging a battery prior to shredding increases the costs of the process. The optimum discharge level has not yet been defined. Depending on the cell chemistry, over-discharging can cause copper to dissolve in the electrolyte. The presence of copper negatively affects the recycling process, as it can contaminate other materials, including the cathode and separator. Additionally, if the battery voltage increases, copper can settle throughout the cells and increase the risk of short circuit and heat outflow.
In Part 2 of this article, we’ll focus on the recycling process and technologies for two battery types: lead-acid and Li-ion.
References[1] JMBS. Our Guide to Batteries; Johnson Matthey Battery Systems: Milton Keynes, UK, 2015. [Google Scholar]