Cross section of a dry cell battery.
Have you ever considered the guts of a battery, the components and assembly, and what makes it perform to your specifications? We’re not talking just about active ingredients and electrochemistry, but physical attributes of the materials and the art involved in cell manufacturing. In general, the cathode drives performance, providing so many milliamp hours/gram within a certain voltage range, thus leading to a cell energy rating (amp hours x volts = Energy in Whr. Power in Watts = amps X volts). But there’s so much more going on that, to define a battery solely by its cathode material is short-changing the other components, active and passive, and the manufacturing process. In this 2-part blog series, we share insights into cause and effect relationships in common batteries: many are chemistry-agnostic and can be applied to almost any battery type.
As usual, let’s define terms so everyone understands the foundation. You can also read our earlier blog posts on Battery Power Density vs Energy Density and on battery terms explained for more description.
- A battery comprises one or more cells, regardless of size; multiple cells in a battery will be linked together by electronic circuitry and/or wiring.
- Primary batteries, such as common alkaline cells, are not rechargeable: use once and recycle. In a few instances, utilizing specialized circuitry, limited recharging is possible, with short working life. Secondary, or rechargeable, cells are capable of hundreds to thousands of charge/discharge cycles: lithium-ion (Li-ion) and nickel metal hydride NiMH) are prime examples.
- Cell voltage is the difference between the cathode and anode potentials (Vcell = Vcat – Van). Primary Li and alkaline chemistries feature anode reactions with Van < 0V. Rechargeable batteries utilize graphite or metal anodes, and Van is usually 500m Wh/g with excellent power ratings. Cells of this type are found in portable tools and hybrid vehicles, and are starting to appear in grid backup settings.
In primary batteries, the anode is an active participant (as defined by the half-cell reactions above), and adds to the cell output voltage. In rechargeable cells like Li-ion and NiMH, the anode graphite and metal alloy provide low voltage storage for Li and H2, respectively. For safety reasons, anode capacity usually is greater than cathode capacity. Excess Li will lead to the growth of highly reactive dendrites which can start runaway reactions with electrolyte solvents (fire or worse). Hydrogen evolution is of course a fire/explosion hazard, but even modest volumes can cause a cell to “balloon” and compromise internal hardware, resulting in shutdown. The Li foil in Li primary batteries raises safety concerns if the cell is breached, but otherwise does not present a problem.
Salt or acid/base solutions allow ion flow from one electrode to the other. KOH is used in many aqueous batteries because it doesn’t produce flammable/explosive hydrogen when in contact with metals (and of course, it’s cheap). Zinc-carbon cells include aqueous NH4Cl and/or ZnCl2 paste, also non-flammable and inexpensive, and primary lithium batteries have a lithium salt dissolved in an organic solvent mix. Lead-acid batteries use H2SO4, and will produce hydrogen if overcharged. Commodity Li-ion cells incorporate LiPF6 dissolved in a complex mix of organic solvents and additives, as many as 15-18 compounds. Depending on the intended usage of the battery, the additives may enhance longevity, permit higher or lower temperature operations, or boost rate capability (power output). The salt is stable above 4V but very reactive with water: it initially forms a phosgene analog and ultimately a mix of hydrofluoric and phosphoric acids. Not nice, but other Li salts simply can’t handle the voltage: your laptop or iPad wouldn’t work without LiPF6.
Next-generation electrolytes may incorporate ionic liquids, unreactive salts that are molten at room temperature, do not burn, have 6+V stability windows, and can be tailored to meet a myriad of needs. Very high purity is a necessity, which greatly increases the cost: performance demands will see ionic liquids in medical and military applications before they can enter the consumer arena. There is also (slowly) growing activity with solid electrolytes, typically a polymer composite with ceramic-like nanoparticle fillers or a salt/ceramic blend with a polymer binder. Although safer than any liquid electrolyte, the down side to solid electrolytes is low conductivity, which requires warming cells to as much as 1000 C for acceptable performance.
Something has to hold electrode powders together – loose particles have limited conductivity, hence poor electrochemistry. In the case of alkaline cells, MnO2 is mixed with a few % graphite (and perhaps a dab of carbon black) and compressed into a hollow cylinder. The soft carbons serve both as a binder and conductive aid. The Zn anode is actually a KOH-Zn slurry with high metal content, so no binder is necessary. Similarly, Ni(OH)2 in NiMH cells is suspended in a high viscosity aqueous alkali hydroxide solution, without binder. Lead-acid batteries use Pb plates for both electrodes.
Li-ion binders are more complex, in part due to the higher voltage environment. Historically, polyvinylidene fluoride (PVDF) was used, both for electrode cohesion and adhesion to the current collector. This polymer has modest solubility in a handful of solvents: N-methylpyrrolidone (NMP) was chosen as the best of several less-than-great options, despite fetal toxicity. Further, NMP is strongly hydrophilic, and water is the enemy in any Li-ion chemistry, so preparing polymer stock solutions and electrode slurries must be done in vacuum or under nitrogen. Lastly, NMP cannot be released into the atmosphere, but must be captured after electrode drying, and then returned to the supplier for purification and re-use.
Over the past few years, a water-based binder system has come into vogue for the Li-ion carbon anode. This is a mix of a water-soluble cellulose derivative and an aqueous styrene co-polymer suspension. Not only does this avoid the toxicity and cost of NMP use, but the carboxyl substituents on the cellulose enhance conductivity and provide a 1-2% boost in cell energy output. Doesn’t sound like much, but marketing teams kill for just such an advantage over the competition. Unfortunately, water and strongly oxidizing cathode materials are incompatible unless exposure is measured in minutes rather than hours. Otherwise, the cathode particle surfaces are passivated with metal hydroxides, and cell performance is diminished. Solving this issue is left to the process engineers…
Why not teflon®? It’s electrochemically unreactive, to ~6V, but also insoluble, so electrodes would have to be constructed with dry mixes or solvent suspensions. Subsequent high-energy mixing operations cause the teflon to fibrillate (not a simple operation), and the electrode particles are held together by a “spiderweb.” This approach is utilized in supercapacitors and primary lithium batteries (similar to a standard alkaline cell).
Hopefully at this point, you appreciate the effect of cathode and anode material, electrolyte and binder on battery performance. In part 2 of this blog series, we discuss the impact of current collector, separator, slurries and conductive aids. As always feel free to contact us for battery development questions or inquiries. You can also check out our battery fundamentals or battery resources sections for useful information.