Manufacturing Process Dependencies and the Performance ofPrismatic 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 andpersonnel 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 ofLithium-Ion Energy Storage Systems

Background-Never before was energy storage so visible, as a principal component of the fundamentalsfor economic development anywhere in the world. Energy storage enables dispatchablepower with relative ease and predictability.

Overall cost and energy density dictates the worthiness and value of the of a storage systemin a particular application. Cost and energy density are so intertwined that is almostimpossible to have a logic conversation about either one separately.

Energy storage systems capable of working as energy ballasts between generation and pointof use are particularly useful since it allows efficient integration of intermittent powergeneration. Solar, wind, and wave technologies provide alternate solutions for energyproduction. These methods of energy production collect and aggregate energy rather thantransforming some stored fuel.

Electrochemistry energy storage systems (batteries and capacitors) provide an effectivesolution for integration of energy collectors (i.e. solar panels) because they store and dispatchgenerated/collected energy. In a sense, the highly reversible charge/discharge processprovides a very low “operation cost” storage.

Electrochemistry energy storage systems cover a broad range of technologies. Eachtechnology 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 tocell phones, laptops, and tablets. Lithium-ion battery technology is now enabling EV andPHEV cars like Tesla, Nissan Leaf, GM Volt, etc.

Lithium-ion battery technology may be the key enabling technology to allow integration ofmore variable resources, wind and solar, to the electric grid. However, in order to become alarge scale enabler, the lithium-ion battery technology has to develop further and come downto the $200 per kWh, which by many is seen as the crucial cost point.

Manufacturing Process Dependencies and the Performance ofPrismatic Large Format Lithium-Ion Cells Page 2

There are three ways to improve the lithium-ion performance: lithium chemistries, the formatand size of the cells, and the manufacturing process. They are all important. The lithium-ionchemistry can be tailored for the application, whether it is focused on power or energy, energydensity or fast recharge, etc.

In this paper, I will focus on the manufacturing process, but it should be noted that format andsize 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 approacheswhen in reality the production options are very limited. There are a handful of anode andcathode 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 substratecarriers. There are systems where nickel substrates are used both for cathode and anodecurrent collectors. Highly pure, contamination free metals are required mandating disciplineand control in the manufacturing processes and handling of the foils.

The raw materials used to manufacture anode and cathode electrodes define the lithium-iontechnology.

Raw materials for anode formulations are evolving from a carbon/graphite base tonanostructured formulations that include crystalline silicon and germanium nanoparticles.These materials are either found in the natural state (natural graphite) or graphitized at hightemperatures. The reaction of lithium carbonate with crystalline titanium oxide produceslithium titanate. This material is produced by at a much lower temperature than thecarbonaceous anode powders.

Raw materials for cathode formulations can be divided into three primary groups; (1) orderedrock salts, (2) spinel and (3) olivine. Most of the manufacturing processes involve conditioningof the materials and some form of calcination.

The physical characteristics of the powders once processed are as important as theelectrochemical characteristics since small variations can contribute to poor cell performance.

The cost of raw materials represents the majority of the battery costs hence the motivationfor 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 energydensity, cycle life, and system efficiency. While significant progress has been made, a greatermanufacturing influence needs presence in the primary material research effort.

Some are of the opinion that a complete new battery technology is required to satisfy theenergy storage target requirements. While there is some reasoning for the position, there is

Manufacturing Process Dependencies and the Performance ofPrismatic Large Format Lithium-Ion Cells Page 3

significant market justification to improve the current products and attempt disruption onexisting battery sectors.

The characterization of the manufacturing process and assessment of the whole systemlifecycle will help to determine the correct vertical integration of processes and distribution ofmanufacturing 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 toowide and yield is too low.That is the main reason that the small 18160 wound cell is used insome large capacity applications.

Cost and yield considerations limit the foil widths. This limitation represents a significantconstraint to the process throughput.

Compared with other roll-to-roll coating processes, electrode coating is a low-speed coatingprocess. The ability to have control in the evaporation of the solvent carriers limits the coatingspeed. The drying process is also a critical factor in the cell performance.

The manufacturing process has 150-180 parameters (materials and process) that contributeto the performance and safety of the product. Introduction of a new material or process to themanufacturing system requires a disciplined and effective methodology for validation.

Standards for sampling and criteria to execute a robust validation of a new material or processin a manufacturing setting are needed.

Today, the manufacturing processes are very discrete and require substantial materialhandling and equipment/facilities lacking synergistic characteristics. Examples are: (1) therequirements 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 adhesionproperties. Significant integration of processes (elimination of processes) or a departure fromthe current manufacturing methodology is required to create disruption in the market sectorespecially in the larger capacity energy storage applications.

Within the same manufacturing lot, the variance in the product (cells) key performancecharacteristics limits the system’s value in the application. The capacity variation affects theeffective system’s capacity; the internal resistance variation affects the effective system’spower rating and so on. Whatever we want to admit it or not, the current lithium-ionmanufacturing output yield a large variance in “product value”.

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The ability to determine the exact performance characteristics of a cell during the formationprocess is a critical need that currently strains the performance of lithium-ion based energystorage systems.

The solution for most of the current manufacturing constraints requires having a clearunderstanding of the process dependencies and how process parameters and capability ofthe various processes affect the performance and cost of the energy storage system in aparticular application.

Process Dependencies- (not all listed)The slurries are mixed in two primary solvent carriers (water and NMP) and the mixing andcoating processes do not vary much between manufacturers. The essential requirement ofthe coating process is to create a defect-free coating volume with particular characteristics ina very consistent manner. The goal is to achieve a consistent narrow distribution of areaspecific capacity in both the anode and cathode so that one can predict the final cell capacityand performance.

There is some reservations on the effectiveness of using water as a solvent carrier in theelectrode manufacturing process. I have been involved in the manufacturing of graphiticanode electrodes, lithium iron phosphate, MNC, and lithium titanate electrodes using wateras the carrier solvent and achieved results compared with those of the NMP process.

After coating, the electrode converting processes assure physical dimensions, moisturecontent and ampacity to create an electrode pair packaged configuration. At some point inthe manufacturing process, one has to fill all electrode void space with electrolyte. Theprocess involves time and pressure assuming that all previous processes yield coatingvolumes with narrow distributions of key characteristics.

A prerequisite to go to larger building blocks, e g large prismatic cells, is to reducemanufacturing variation.

The Cpk of a particular key performance parameter at a specific process in the productionsystem is dependent of the performance of the previous processes. Processes with Cpkvalues of 1.00 can’t assure cell performance yet many of the current processes are belowsuch values. Some of these issues are related to discipline and culture, but in general themanufacturing control of lithium-ion is complex and difficult.

The process control capabilities of each manufacturing process and the understanding of thedependencies between processes allow, not only to produce a robust product but also theimplementation 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, tapdensity, energy density, Gurley rate, carbon content, contaminants.

Manufacturing Process Dependencies and the Performance ofPrismatic 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, volumeresistanceCalendering- Electrode thickness, surface roughness, volume resistance, porosityStamping/ Slitting- Edge quality, size, embedded particles,Stacking/Winding- Electrode position,Termination- AmpacityPackaging- Surface contact pressure, handling defects, dielectric strength, seal integrityDrying- Moisture content, separator shrinkageFilling- 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.

Manufacturing Process Dependencies and the Performance ofPrismatic Large Format Lithium-Ion Cells Page 6

The complexity, control and efficiency of a lithium-ion energy storage system depend muchon the configuration of the cells. In general the most simplistic system would use a singlewound cell. The next level of complexity would be to connect individual wound cells in seriesto provide a higher voltage. To increase the current capacity, one would combine multipleseries 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 performancecharacteristics dictate the efficiency and usefulness of the energy storage system. The controland discipline on a throughly assessed manufacturing process substantially improve thesafety of the product.

The process control capabilities of each manufacturing process and the understanding of thedependencies between processes allow, not only to produce a robust product but also theimplementation of continuous improvements that ultimately make the systems cost efficient.