Next generation systems
As lithium ion intercalation systems are reaching their limitations in capacity many electrochemists are returning to resolving the challenges of redox systems. Advances in particle encapsulation, SEI formation, ion selective membranes, dendrite plating control, and nano- materials are offering new life to these systems.
Carbons and Graphenes
Carbons play a critical role in all battery electrodes. In choosing a carbon one must first define the role of the carbon: conductivity, electrolyte absorption, coating aid, process aid such as lubricity and rheology, powder flow, contributor to the electrode pore structure, oxygen reduction, cathoylte reaction sites, etc. For hard carbons, consider such physical properties as: aggregate and particle size, surface area, functional surface groups as defined by Boehm titration, structure, micro, meso, and macro pores, pH, absorptivity of liquids, conductivity, and contribution to electrode integrity. When considering graphitic carbons, the transition from single layer graphene to multi-layer graphenes to graphite to surface groups to graphene oxide is an important consideration. Of course, the discussion becomes more detailed when the carbon plays a role in the electrochemical reaction, such as in a lithium ion anode involving interaction, or when it serves as a catalytic site, such as for oxygen reduction, or as a reaction site for sulfur reduction such as in lithium sulfur batteries.
Membranes and Separators
For the most part, battery separators can be grouped into two categories; non-wovens and microporous membranes. The choice of a separator for each battery system must take into consideration such factors as cost, thickness, processibility, formability, pore size, tortuosity of pores, tensile strength, melt point, range of operating temperature stability, puncture resistance, dielectric strength, particle sizes of the anode and cathode materials, and stresses developed during product discharge.
Typically, carbon zinc and alkaline batteries use non-woven separators. In these cases, cost is an important design consideration, and the high conductivity of the electrolyte enables thicker separator materials. However, the thickness of the separator also defines the gap or distance between electrodes, which can impact the cell’s ability to support high current densities. Pores are not well defined and are typically on the order of several microns, while the thicknesses are often several hundreds of microns.
In contrast, lithium cells require thin microporous separators based on the low conductivity of organic electrolytes. These microporous separators more closely resemble membranes with a thickness of 25 microns or less with pore diameters in the hundredths of microns. They most often are polyolefins consisting of polyethylene or polypropylene.
In selecting a separator for a particular battery design, it’s also important to consider its stability in the electrolyte, ability to wet out quickly by the electrolyte, ability to maintain electrolyte within its pore structure throughout the battery’s life, and the impact it has on ionic mobility of the electrolyte salt. Since separator pores are seldom channels straight through the separator, the difference in true ion path versus the separator thickness is referred to as tortuosity. A certain amount of tortuosity is desired to prevent shorting due to particle penetration and soft shorts from dendritic bridging between electrodes, while maintaining low tortuosity is often required to achieve the best high rate performance. Therefore, striking the right balance of pore size and structure is an important design consideration.
The trend today is toward more value-added membranes such as ceramic coatings, ion selectivity, coatings to improve electrolyte absorption and reduce impedance, and dendrite blocking.
The utilization of designed experiments is an absolute must in optimizing today’s batteries, whether it’s a need to establish the optimal mean value for a variable, set acceptable parameter ranges, reduce process waste, optimize performance based on ranking, optimize formulations for processing / performance / costs, surface response outcome modeling, or understand how variables interact and influence results. A FAST, Function Analysis System Technique, diagram is almost always recommended before attempting to define the variables to be considered in a designed experiment. Time, cost, and performance are all optimized by a thorough upfront designed approach to product design and problem solving. Today’s software can run hundreds of thousands of Monte Carlo scenarios in seconds providing insight that empirically could take months or years to establish.
High Slurry Solids Coating
Manufacturing of battery electrodes, while relying upon common coating techniques, can be extremely challenging due to formulation limitations, raw material limitations, the need to limit process aids, their high solids content, high loading requirements, consistency of loadings to balance anode to cathode capacity ratios, limitations in compatible polymer binders which then define acceptable solvents, the need for high density or particle packing, as well as a necessity to create patterns both in the web direction and cross web direction. Common techniques often start with relatively crude tape casting, then progressing to: rotary screen, three roll reverse, comma coating, and slot die.
One of the great challenges of defining an electrode formulation involves how the physical properties of the active materials influence and impact the requirements of the non- active components such as: conductors, polymers / binders, rheology aids, solvents, mix processes, the need for particle milling or aggregate reduction, filtration, and potential impurities from equipment wear. One of the critical properties of the raw materials can be directly associated with particle surface area. For example, common battery materials such as LiCo2, LiMnO2, Sulfur, FeS2, etc. are typically less than one m2/gm. While carbons can range from 20 to 2000 m2/gm, other active material such as MnO2 may have a surface area of 30 and CFX maybe 250 or greater. Years of empirical work on electrode formulations has resulted in transfer functions which, when based off of a combination of BET surface area and DBP (DiButyl Phthalate) absorption, can define a starting point for polymer content, while the conductivity of the active materials can be used to define the amount and carbon preference.
For pattern or patch coating it’s important to measure the slurry rheology over a shear range that represents the coating process utilized. Since most battery coating formulations are shear sensitive or non-Newtonian, a single viscosity value can be very misleading. Additionally, and particularly for cross web mass free zones the type and speed (milliseconds) of mechanical, servo and or servo- mechanical processes, as well as fluid flow control, must be correlated to the web speed and length of the mass free zone. Lastly, it’s important to take into account the surface tension of the coating slurry and the substrate.
Powder compaction and particulate coating
Dry powder compaction electrodes, while one of the oldest techniques, is making a comeback as companies attempt to eliminate the environmental and cost issues associated with solvent based processes. Additionally, companies are attempting to avoid the negative attributes of polymers which coat the surface of active materials, negatively impacting particle to particle conductivity as well as ion diffusion and mobility limitations. As electrode structures become more dependent on nano particulate coatings on the surface of micron size active materials (particulate coating), dry powder processing can be a more effective approach to distributing and maintaining the weak Van der Waal bonds. Of course, certain electrochemical systems require dry powder processing since the choice of compatible binders may be limited to non-soluble polymers such as Teflon (Li-MnO2, Li-SOCl2, Metal-Air, etc.). The main limitation to this type of electrode fabrication is that many of the process controls are coupled with the electrode properties; these include loading, thickness, packing or density. From a process standpoint, the size / diameter of the working rolls, their surface roughness, powder flow properties, gap and or force define the electrode characteristic. As a result, it’s difficult, if not impossible, to control electrode thickness while controlling electrode loading and packing. Despite these limitations, with a thorough understanding of these interactions, acceptable electrodes can be produced by this solvent free method of fabrication.
SEI or Solid Electrolyte Interfaces are becoming one of the hottest areas of battery design fundamentals. Once limited to mostly lithium ion anode studies, today it is being viewed as the solution to improved efficiency, higher voltage compatibilities, reduced corrosion, improved cycle life, increased operating temperature windows, improved shelf life, impedance control, a means to control reaction product solubility, selective ionic diffusion control, a means to expanded electrolyte options, and the list goes on. Once often over looked in aqueous battery systems, the transition to mercury free design stressed the importance of stabilizing the surface of zinc from corrosion and hydrogen generation, while balancing impedance contributions associated with the passivation SEI. Whether referred to as particle encapsulation, electrolyte additives, or corrosion and passivation, there is no denying the significance of SEI films and their contribution and role in electrochemical optimization. The lithium metal battery would not be possible without the formation of a stable SEI passivation layer on its surface. Advances in material science and nano-chemistry, as well as coatings and encapsulation technologies will make this a hot topic of study and enable continued optimization of existing and future product designs.