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| Fig. 1: Schematic showing our idealized two-phase mixture in its initial state (top) and unmixed final state (bottom). (Image source: P. Carpenter) |
Lithium-ion batteries (LIBs) are ubiquitous in everyday life. From electric toothbrushes to smartphones to power tools, the high energy-density and long lifespan of LIBs has allowed them to power many common handheld devices. Increasingly, lithium-ion battery cells are also being implemented into more energy intensive applications ranging from electric vehicles to grid-scale energy storage. In response to this growing demand for and scale of lithium ion batteries in a wide variety of applications, LIB sales have grown steadily: a 2016 forecast, for example, predicted that the market for lithium ion batteries would grow 10.4% to achieve a total market value of USD $50.6 billion by 2024. [1]
As more lithium-ion batteries enter circulation into various applications, recycling of spent batteries has been considered as an alternative source for raw LIB materials. It is currently estimated that less than 5% of all lithium ion batteries produced are recycled today, but increasing demand for LIBs, coupled with an increasing amount of spent LIB waste, provides a compelling motivation for the development of battery recycling. [1] However, LIB recycling is a complex process, requiring both investigation into fundamental science and an understanding of logistical and economic constraints. Here, we first develop a qualitative physical model to understand the physics of LIB recycling, and then evaluate LIB recycling within the current logistic and economic barriers facing the industry.
Though quantitative energy modeling of a battery recycling process would require a careful breakdown of each step, a simplistic system can be used to gain qualitative intuition for the energy involved with battery recycling. Consider a system of atoms, represented by colored circles, shown in the top half of Fig. 1, with some initial amount of red atoms randomly-dispersed among gray atoms. Now, suppose we want to sort these atoms such that the red atoms are isolated from the gray atoms and concentrated together to form two unmixed phases. What is the energy required to achieve this process?
We can approach this question by describing the change in entropy of the atoms from the initial to final state. [2] Here, entropy can be thought of as a measure of disorder, or randomness, within a system. Our initial state, with a random assortment of red and gray atoms, will have a much higher entropy than our final state of unmixed and well-ordered red and gray atoms. Because we must go from a state of high entropy to a state of low entropy, the second law of thermodynamics dictates that some energy, or work, must be inputted into the system in order to demix our red and gray atoms. Using the ideal gas law and the laws of thermodynamics, the following expression can be derived for the work, W, required to demix the red and gray atoms from each other as a function of the initial fraction of red atoms in the solution (x):
| W | = | - RT [ xln(x) + (1-x)ln(1-x) ] |
where R is the universal gas constant (8.314 J K-1 mol-1) and T is the absolute temperature (in Kelvin). [2] This equation then describes the energy required to demix the red atoms per mole of the initial solution. However, because our desired product is red atoms, it is more useful to consider the energy required to demix per mole of red atoms instead. Dividing by the unit fraction of red atoms, we then obtain the normalized work, Wnorm, again as a function of the initial fraction of red atoms:
| Wnorm | = | W/x |
If we now frame our equation for the work of demixing in the context of lithium ion batteries, we can use the above equation to compare the work required to extract lithium metal in mining to removing it from an existing battery. By allowing lithium to represent our red atoms and all other components of the ore or battery to be our gray atoms (in this simplistic model, it does not matter that the everything else of the gray atoms will be in and of itself a mixture), demixing then represents the separation of lithium from the source material into its raw form.
The concentration of lithium atoms in lithium ore ranges on average from 0.5-2%. [3] Using the midpoint of this range and the above normalized equation for the work of extraction, we can calculate that the work required to "demix," or extract one mole of lithium from ore, would be 1.34 × 104 J/mole at ambient temperature (300K). By contrast, the concentration of lithium in LIBs is on average 5-7%. [4] Again using the midpoint of this range, we find that the work required to "demix" a mole of lithium from an LIB is 9.44 × 103 J/mole. By comparing these two values for work, our simplistic model suggests that energy consumption for LIB recycling could be roughly comparable to energy required for mining. However, we must also provide a caveat: real battery recycling processes often require many types of work including thermal energy (high temperatures), mechanical grinding, and chemical reduction. As such, the actual work required to recycle one mole of lithium is likely higher than our calculated demixing value.
Despite a roughly comparable energy cost to mining, however, the economic costs of large-scale LIB recycling remain far too high to be profitable. As such, it is crucial to also evaluate the potential economic drivers and barriers to increasing LIB recycling rates from the current baseline of 5%. To do so, we will compare LIB recycling to the development of lead acid battery recycling in the United States. With a recovery and recycling rate of over 99%, lead acid batteries are often used as a case study for the successful development of battery recycling programs. [5] There are three defining characteristics that enabled this high rate, but that limit LIB recycling today:
Complexity and profitability of processes: Lead acid batteries have a relatively simple chemistry compared to lithium-ion batteries, and have existed as a mature technology for far longer. As such, existing lead acid battery recycling processes are far simpler and more profitable than proposed LIB recycling processes. [5] Of existing processes, it is currently thought that direct cathode recycling - in which cathodes are mechanically removed from spent LIB materials - is the only potentially profitable process at a large scale. [5]
Regulatory incentives and the distributed nature of the resource: While the European Union and some US states have set targets for LIB end-of-life collection, there exist no other regulatory incentives for lithium ion battery recycling. By contrast, US federal law mandates the recycling of lead acid batteries, and many states have adopted exchange mandates, in which the sale of a new lead acid battery requires the exchange of a spent one. This particular incentive is crucial in solving a fundamental problem of battery recycling: the distributed nature of spent batteries, which often sit unused and idle in consumers homes. Without incentivizing an efficient method of collection, such as exchange mandates, the cost of collecting and transporting spent LIBs to recycling plants is prohibitively high. [5]
Public compliance and awareness: Strict federal regulations, coupled with exchange mandates, have also ensured that lead acid battery consumers are compliant with incentives for recycling. [5] However, because LIB recycling currently lacks regulatory support, there exists no incentive for LIB consumers to properly recycle their spent batteries. It is, after all, far easier to leave it on an empty shelf than it is to transport it to a recycling center.
Ultimately, a 2019 techno-economic analysis published in Nature estimated that the cost of some LIB recycling processes might eventually decrease to equal those of mining raw materials, making recycling a profitable and competitive source for raw LIB materials. [5] However, this analysis also revealed a heavy reliance on economies of scale to achieve a profitable business model. [5] As such, until the above hurdles to scaling up LIB recycling can be addressed by regulatory action, improved process efficiency and increased public support, the current state of lithium-ion battery recycling is not an economically viable process.
© Peter Carpenter. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] O. Velázquez-Martinez et al., "A Critical Review of Lithium-Ion Battery Recycling Processes from a Circular Economy Perspective," Batteries 5, 68, (2019).
[2] F. Reif, Fundamentals of Statistical and Thermal Physics (McGraw-Hill, 1965), p. 243.
[3] P. Meshram, P. D. Pandey and T. R. Mankhand, "Extraction of Lithium From Primary and Secondary Sources by Pre-Treatment, Leaching and Separation: A Comprehensive Review," Hydrometallurgy 150, 192 (2014).
[4] X. Zheng et al., "A Mini-Review on Metal Recycling from Spent Lithium Ion Batteries," Engineering 4, 361 (2018).
[5] R. E. Ciez and J. F. Whitacre, "Examining Different Recycling Processes for Lithium-ion Batteries," Nat. Sustain. 2, 148 (2019).