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Current and future lithium-ion battery manufacturing
2022-10-29

Lithium-ion batteries (LIBs) have become one of the main energy storage solutions in modern society. The application fields and market share of LIBs have increased rapidly and continue to show a steady rising trend. The research on LIB materials has scored tremendous achievements. Many innovative materials have been adopted and commercialized by the industry. However, the research on LIB manufacturing falls behind. Many battery researchers may not know exactly how LIBs are being manufactured and how different steps impact the cost, energy consumption, and throughput, which prevents innovations in battery manufacturing. Here in this perspective paper, we introduce state-of-the-art manufacturing technology and analyze the cost, throughput, and energy consumption based on the production processes. We then review the research progress focusing on the high-cost, energy, and time-demand steps of LIB manufacturing. Finally, we share our views of challenges in LIB manufacturing and propose future development directions for manufacturing research in LIBs.

Subject areas

Electrochemical Energy Storage
Industrial Chemistry
Energy Storage
Industrial Processing of Material
Energy Materials

Introduction

Lithium-ion batteries (LIBs) have been widely used in portable electronics, electric vehicles, and grid storage due to their high energy density, high power density, and long cycle life. Since Whittingham discovered the intercalation electrodes in the 1970s, Goodenough et al. developed some key cathode materials (layered, spinel, and polyanion) in the 1980s and the 1990s, and Yoshino created the first safe, production-viable LIB with the combination of LiCoO2 as the cathode and carbon/graphite as the anode, much progress in LIBs have been made in terms of cost, energy density, power density, safety, and cycle life (Whittingham, 1976; Mizushima et al., 1980; Thackeray et al., 1983; Padhi et al., 1997). For example, the cost of LIBs has dropped from over $1,000/kWh in the early 2000 to ∼$200/kWh currently. At the same time, the specific energy density of LIBs has been increased from 150 Wh/kg to ∼300 Wh/kg in the past decades. Although beyond LIBs, solid-state batteries (SSBs), sodium-ion batteries, lithium-sulfur batteries, lithium-air batteries, and multivalent batteries have been proposed and developed, LIBs will most likely still dominate the market at least for the next 10 years.

Currently, most research studies on LIBs have been focused on diverse active electrode materials and suitable electrolytes for high cutoff voltage applications, especially the nickel-rich and/or cobalt-free cathode materials and Si or Li metal anode materials and their associated electrolytes. Progress in LIB manufacturing lags behind and not much progress has been made, although manufacturing contributes about 25% of the cost of LIBs (Kwade et al., 2018). Currently, the manufacturing of LIBs still needs to go through slurry mixing, coating, drying, calendering, slitting, vacuum drying, jelly roll fabrication (stacking for pouch cells and winding for cylindrical and prismatic cells), welding, packaging, electrolyte filling, formation, and aging, a multi-staged process being adopted by industry.

In this perspective paper, we first evaluate each step of the current manufacturing process and analyze their contributions in cost, energy consumption, and throughput impacts for the entire LIB production. Then we summarize the recent progress on the advancement of LIB manufacturing and the challenges and the potential impacts of these new technologies. Finally, we provide our perspectives on future LIB manufacturing. We hope that such a paper helps promote more collaboration between the academia and industry with the ultimate goal to solve some key issues of LIB manufacturing, which may eventually result in increasing the production efficiency and lowering the cost and energy consumption of LIBs.

Current manufacturing processes for LIBs

LIB industry has established the manufacturing method for consumer electronic batteries initially and most of the mature technologies have been transferred to current state-of-the-art battery production. Although LIB manufacturers have different cell designs including cylindrical (e.g., Panasonic designed for Tesla), pouch (e.g., LG Chem, A123 Systems, and SK innovation), and prismatic (e.g., Samsung SDI and CATL), the cell manufacturing processes are very similar.

Figure 1 introduces the current state-of-the-art battery manufacturing process, which includes three major parts: electrode preparation, cell assembly, and battery electrochemistry activation. First, the active material (AM), conductive additive, and binder are mixed to form a uniform slurry with the solvent. For the cathode, N-methyl pyrrolidone (NMP) is normally used to dissolve the binder, polyvinylidene fluoride (PVDF), and for the anode, the styrene-butadiene rubber (SBR) binder is dissolved in water with carboxymethyl cellulose (CMC). The slurry is then pumped into a slot die, coated on both sides of the current collector (Al foil for cathode and Cu foil for the anode), and delivered to drying equipment to evaporate the solvent. The common organic solvent (NMP) for cathode slurry is toxic and has strict emission regulations. Thus a solvent recovery process is necessary for the cathode production during drying and the recovered NMP is reused in battery manufacturing with 20%–30% loss (Ahmed et al., 2016). For the water-based anode slurry, the harmless vapor can be exhausted to the ambient environment directly. The following calendering process can help adjust the physical properties (bonding, conductivity, density, porosity, etc.) of the electrodes. After all these processes, the finished electrodes are stamped and slitted to the required dimension to fit the cell design. The electrodes are then sent to the vacuum oven to remove the excess water. The moisture level of the electrodes will be checked after drying to ensure the side reaction and corrosion in the cell are minimized.

Figure 1. Schematic of LIB manufacturing processes

After the electrodes are well prepared, they are sent to the dry room with dried separators for cell production. The electrodes and separator are winded or stacked layer by layer to form the internal structure of a cell. The aluminum and copper tabs are welded on the cathode and anode current collector, respectively. The most common welding method is ultrasonic welding, and some manufacturers may choose resistance welding for their cell design. The cell stack is then transferred to the designed enclosure, which does not have a consistent standard currently. Each manufacturer has their preference depending on the purpose of the cells. The enclosure is filled with electrolyte before the final sealing and completes the cell production.

Before delivering the cells to the end product manufacturers, the electrochemistry activation steps are applied to these cells to enable operation stability. A stable solid-electrolyte interface (SEI) layer can prevent the irreversible consumption of electrolyte and protect the anode from overpotential during fast charging, which can result in forming Li dendrites (Li et al., 2019). The formation and aging process starts from charging the cells to a relatively low voltage (e.g., 1.5V) to protect the copper current collector from corrosion, followed by a rest session for electrolyte wetting. The cells are charged/discharged under a low rate such as C/20, and then the rate will be gradually increased to ensure a stable SEI layer on the surface of the anode (Wood et al., 2019). The gas generated from the formation process needs to be discharged for safety concerns. After or during formation cycles, the cells are stored on the aging shelves for complete electrolyte wetting and SEI stabilization. Another degassing step is arranged before the cells are finally sealed for future applications. Depending on the formation protocol and aging temperature, this step normally lasts several weeks.

The estimate of the cost, throughput, and energy consumption for these manufacturing steps is critical to help determine the steps that need the most research and innovation. Therefore, more research efforts can be focused on these topics. Table 1 and Figure 2A show the breakdown of manufacturing cost calculated by the BatPac model from Argonne National Laboratory. The model was based on a 67-Ah LiNi0.6Mn0.2Co0.2O2 (NMC622)/graphite cell, 100,000 EV battery packs/year plant (Nelson et al., 2019). The electrode coating, drying, cell formation, and aging contributed to 48% of the entire manufacturing cost. These high capital investments and labor-intense processes are the most urgent fields that need to be studied. The cost saving will be significant if the laboratory innovations can be transferred to these manufacturing processes.



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