Manufacturing Process Dependencies and the Performance of Prismatic Large Format Lithium-Ion Cells
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This document discusses lithium-ion battery manufacturing processes and their impact on battery performance. It notes that manufacturing speed, variation, and yield are currently too low and contribute to high costs. Improving understanding of process dependencies could help address constraints and optimize performance and costs to enable lithium-ion batteries for large-scale energy storage applications. The document focuses on manufacturing processes for electrode coatings, conversion processes, and electrolyte filling and how consistency and control at each stage impacts final battery characteristics and performance.
Manufacturing Process Dependencies and the Performance of Prismatic Large Format Lithium-Ion Cells
- 1. Manufacturing Process Dependencies and the Performance of Prismatic Large Format Lithium-Ion Cells Page 1 By Antonio Reis. Antonio Reis is a professional in equipment design and industrial maintenance for more than 25 years. Antonio Reis has provided management, manufacturing development, maintenance services and personnel training to various types of industries such as Metallurgical, Meat Packing, Food & Beverage, Petrochemical, Automotive, Battery, Converting and Semiconductor. June 2015 Manufacturing Process Dependencies and the Performance of Lithium-Ion Energy Storage Systems Background- Never before was energy storage so visible, as a principal component of the fundamentals for economic development anywhere in the world. Energy storage enables dispatchable power with relative ease and predictability. Overall cost and energy density dictates the worthiness and value of the of a storage system in a particular application. Cost and energy density are so intertwined that is almost impossible to have a logic conversation about either one separately. Energy storage systems capable of working as energy ballasts between generation and point of use are particularly useful since it allows efficient integration of intermittent power generation. Solar, wind, and wave technologies provide alternate solutions for energy production. These methods of energy production collect and aggregate energy rather than transforming some stored fuel. Electrochemistry energy storage systems (batteries and capacitors) provide an effective solution for integration of energy collectors (i.e. solar panels) because they store and dispatch generated/collected energy. In a sense, the highly reversible charge/discharge process provides a very low “operation cost” storage. Electrochemistry energy storage systems cover a broad range of technologies. Each technology has advantages and challenges depending on operational requirements, portability, life-cycle, and cost. Lithium-Ion Technologies- Lithium-ion battery technology has enabled the mobile revolution from Sony Walkman to cell phones, laptops, and tablets. Lithium-ion battery technology is now enabling EV and PHEV cars like Tesla, Nissan Leaf, GM Volt, etc. Lithium-ion battery technology may be the key enabling technology to allow integration of more variable resources, wind and solar, to the electric grid. However, in order to become a large scale enabler, the lithium-ion battery technology has to develop further and come down to the $200 per kWh, which by many is seen as the crucial cost point.
- 2. Manufacturing Process Dependencies and the Performance of Prismatic Large Format Lithium-Ion Cells Page 2 There are three ways to improve the lithium-ion performance: lithium chemistries, the format and size of the cells, and the manufacturing process. They are all important. The lithium-ion chemistry can be tailored for the application, whether it is focused on power or energy, energy density or fast recharge, etc. In this paper, I will focus on the manufacturing process, but it should be noted that format and size of the battery cells are tightly linked to the manufacturing process. In the lithium-ion sector, it appears that there is a large variety of manufacturing approaches when in reality the production options are very limited. There are a handful of anode and cathode chemistries (formulations) all of them coated on metallic substrates (copper, aluminum and nickel). Usually, anode coatings use copper foils, and cathode coatings use aluminum as substrate carriers. There are systems where nickel substrates are used both for cathode and anode current collectors. Highly pure, contamination free metals are required mandating discipline and control in the manufacturing processes and handling of the foils. The raw materials used to manufacture anode and cathode electrodes define the lithium-ion technology. Raw materials for anode formulations are evolving from a carbon/graphite base to nanostructured formulations that include crystalline silicon and germanium nanoparticles. These materials are either found in the natural state (natural graphite) or graphitized at high temperatures. The reaction of lithium carbonate with crystalline titanium oxide produces lithium titanate. This material is produced by at a much lower temperature than the carbonaceous anode powders. Raw materials for cathode formulations can be divided into three primary groups; (1) ordered rock salts, (2) spinel and (3) olivine. Most of the manufacturing processes involve conditioning of the materials and some form of calcination. The physical characteristics of the powders once processed are as important as the electrochemical characteristics since small variations can contribute to poor cell performance. The cost of raw materials represents the majority of the battery costs hence the motivation for the development of materials with improved energy density and or cycle life. Looking Forward- Understanding of lithium-ion manufacturing is critical for research aimed at improving energy density, cycle life, and system efficiency. While significant progress has been made, a greater manufacturing influence needs presence in the primary material research effort. Some are of the opinion that a complete new battery technology is required to satisfy the energy storage target requirements. While there is some reasoning for the position, there is
- 3. Manufacturing Process Dependencies and the Performance of Prismatic Large Format Lithium-Ion Cells Page 3 significant market justification to improve the current products and attempt disruption on existing battery sectors. The characterization of the manufacturing process and assessment of the whole system lifecycle will help to determine the correct vertical integration of processes and distribution of manufacturing assets for optimization of product performance and cost. Manufacturing Constraints- There are three main challenges in the manufacturing process: speed, variation, and yield. The truth of the matter is that in the present state-of-the-art manufacturing variation is too wide and yield is too low.That is the main reason that the small 18160 wound cell is used in some large capacity applications. Cost and yield considerations limit the foil widths. This limitation represents a significant constraint to the process throughput. Compared with other roll-to-roll coating processes, electrode coating is a low-speed coating process. The ability to have control in the evaporation of the solvent carriers limits the coating speed. The drying process is also a critical factor in the cell performance. The manufacturing process has 150-180 parameters (materials and process) that contribute to the performance and safety of the product. Introduction of a new material or process to the manufacturing system requires a disciplined and effective methodology for validation. Standards for sampling and criteria to execute a robust validation of a new material or process in a manufacturing setting are needed. Today, the manufacturing processes are very discrete and require substantial material handling and equipment/facilities lacking synergistic characteristics. Examples are: (1) the requirements for a very dry manufacturing environment yet with clean room characteristics, and (2) multiple wind/rewind of a double-sided coated product with relative low adhesion properties. Significant integration of processes (elimination of processes) or a departure from the current manufacturing methodology is required to create disruption in the market sector especially in the larger capacity energy storage applications. Within the same manufacturing lot, the variance in the product (cells) key performance characteristics limits the system’s value in the application. The capacity variation affects the effective system’s capacity; the internal resistance variation affects the effective system’s power rating and so on. Whatever we want to admit it or not, the current lithium-ion manufacturing output yield a large variance in “product value”.
- 4. Manufacturing Process Dependencies and the Performance of Prismatic Large Format Lithium-Ion Cells Page 4 The ability to determine the exact performance characteristics of a cell during the formation process is a critical need that currently strains the performance of lithium-ion based energy storage systems. The solution for most of the current manufacturing constraints requires having a clear understanding of the process dependencies and how process parameters and capability of the various processes affect the performance and cost of the energy storage system in a particular application. Process Dependencies- (not all listed) The slurries are mixed in two primary solvent carriers (water and NMP) and the mixing and coating processes do not vary much between manufacturers. The essential requirement of the coating process is to create a defect-free coating volume with particular characteristics in a very consistent manner. The goal is to achieve a consistent narrow distribution of area specific capacity in both the anode and cathode so that one can predict the final cell capacity and performance. There is some reservations on the effectiveness of using water as a solvent carrier in the electrode manufacturing process. I have been involved in the manufacturing of graphitic anode electrodes, lithium iron phosphate, MNC, and lithium titanate electrodes using water as the carrier solvent and achieved results compared with those of the NMP process. After coating, the electrode converting processes assure physical dimensions, moisture content and ampacity to create an electrode pair packaged configuration. At some point in the manufacturing process, one has to fill all electrode void space with electrolyte. The process involves time and pressure assuming that all previous processes yield coating volumes with narrow distributions of key characteristics. A prerequisite to go to larger building blocks, e g large prismatic cells, is to reduce manufacturing variation. The Cpk of a particular key performance parameter at a specific process in the production system is dependent of the performance of the previous processes. Processes with Cpk values of 1.00 can’t assure cell performance yet many of the current processes are below such values. Some of these issues are related to discipline and culture, but in general the manufacturing control of lithium-ion is complex and difficult. The process control capabilities of each manufacturing process and the understanding of the dependencies between processes allow, not only to produce a robust product but also the implementation of continuous improvements that ultimately make the systems cost efficient. Here are some of the processes and related the basic parameters and characteristics: Raw Materials- Particle shape(s), particle size distribution, particle shape distribution, tap density, energy density, Gurley rate, carbon content, contaminants.
- 5. Manufacturing Process Dependencies and the Performance of Prismatic Large Format Lithium-Ion Cells Page 5 Mixing- sequence, add rate, temperature, viscosity, rheology, % solids, entrapped air, dispersion. Coating- Thickness variation, porosity, adhesion, surface roughness, surface defects, volume resistance Calendering- Electrode thickness, surface roughness, volume resistance, porosity Stamping/ Slitting- Edge quality, size, embedded particles, Stacking/Winding- Electrode position, Termination- Ampacity Packaging- Surface contact pressure, handling defects, dielectric strength, seal integrity Drying- Moisture content, separator shrinkage Filling- Electrolyte volume, wetting distribution, Formation- Capacity, 1st cycle efficiency, gas generation, Product validation- Internal resistance, cell impedance, voltage retention, safety performance Figure 1 Conclusions- There are significant discussion and opinions on what is the best lithium-ion cell format.
- 6. Manufacturing Process Dependencies and the Performance of Prismatic Large Format Lithium-Ion Cells Page 6 The complexity, control and efficiency of a lithium-ion energy storage system depend much on the configuration of the cells. In general the most simplistic system would use a single wound cell. The next level of complexity would be to connect individual wound cells in series to provide a higher voltage. To increase the current capacity, one would combine multiple series strings in parallel each string able to able to disconnect from the system. The quality of the cell and the narrow distribution of the various key performance characteristics dictate the efficiency and usefulness of the energy storage system. The control and discipline on a throughly assessed manufacturing process substantially improve the safety of the product. The process control capabilities of each manufacturing process and the understanding of the dependencies between processes allow, not only to produce a robust product but also the implementation of continuous improvements that ultimately make the systems cost efficient.