3 Considerations for Maximizing Battery Electrode Capacity or Power

A lot of factors influence battery electrode optimization. But what is electrode optimization in the first place– it is optimizing your battery for power or energy, whichever metric is most important to your application.

If you have an application where you need a lot of power, then your electrodes must be optimized to deliver that high current rate. This means you need sufficient conductive aid and enough void volume for electrolyte to be able to flow. However in the process, you consume space and reduce your energy density.

Likewise if you have an application where you need high energy density, you would maximize your active material packing and minimize conductive aid and electrolyte volume- which means you will now not be optimized for power.

 Active material inputs need to be maximized while maintaining adequate electrical conductivity and ionic conductivity networks and assuring structural integrity with appropriate binders. The following discussion considers the relative volumes of the components of the electrode. Too much conductor or too much electrolyte takes away from space for active materials. Different types of binder for maintaining structural integrity may also differ significantly in the amount of electrode volume they occupy.

While optimal compositions can be developed relatively straightforwardly for distribution of monodisperse (all particles the same size) spherical particles, the real world dictates that such “pure” concepts be modified for the polydisperse (particle sizes are not uniform) non-spherical particles of most electrode materials. With mono-disperse particles theoretical packing calculations are easy- it is easier to control and manage packing in a given volume. With poly-disperse particles on the other hand, packing is more difficult to control, but higher packing densities can be achieved where you optimize for energy density.

We discuss 3 important considerations in this blog post for optimizing your battery electrodes.

1 – Conductivity Networks are Critical to Electrode Function

To function usefully, an electrode must have both electronic and ionic communication to the site of each electrochemical half-reaction. This means that for high efficiency utilization of active materials, attention must be paid to the electrical and ionic conductivity networks within the electrode structure, so that each particle of active material takes part in the reaction.

In a standard lithium ion cell, the discharge process can be described as:

CnLi ⇒ Li+ + e + nC at the anode

Li+ + e + 2Li0.5CoO2 ⇒ 2LiCoO2 at the cathode

This can be written more generally as:

Li in Anode host material ⇒ Li+ in electrolyte + e to external circuit

===============separator===============

Li+ in electrolyte + e from external circuit ⇒ Li in Cathode host material

Li+ must be able to move from one electrode to the other in an electrolyte path through the separator and through the body of each electrode structure to each particle of active material.

Similarly, e- must be able to move in the external circuit and through the body of each electrode to each particle of active material.

The electrochemical half-reaction typically takes place on the surface of the host material. To maintain the discharge or charge reaction, lithium atoms and electrons must be able to migrate to/from the surface of each active material particle to the “deepest” part of the particle, farthest from the surface where the reaction takes place. 

Considerations of both particle-particle (inter-particle) and within-particle (intra-particle) electronic and ionic conductivities contribute to the design of efficient electrodes.

2 – Electrode Structure and Design 

A typical electrode composition will include active material, electronic conductor to distribute and collect current from individual active material particles, electrolyte to allow the ion transfer necessary to and from the active material particles to support the electrochemical reaction, and often a binder to preserve the electrode structure during the volume changes that occur with charge or discharge.

A good electrode design will provide maximum energy and/or power by maximizing the amount of active material while still satisfying the ionic and electronic network requirements for efficient use of the active material. This requires there be good low-resistance electrical and ionic networks to support the electrochemical reaction on each active material particle. The best conceptual framework for engineering this balance is in volumetric analysis:  looking at how the total electrode volume is partitioned for active material, conductive material, ionic electrolyte, and binder. Most active materials undergo volume changes during discharge and charge, whereas the conductor does not change its volume. In aqueous electrolyte systems, H2O is usually a participant in the electrochemical half-reactions, so that H2O transport, and sometimes consumption, becomes a factor. It is important to design the electrode structure so the volumes of conductor and electrolyte will be adequate for the electrode in both its charged and its discharged states.

Most active materials come in roughly spherically shaped particles with a limited range of particle sizes to avoid size segregation phenomena during processing. Experience has shown that random packing of such materials as is usually achieved in high-speed manufacturing produces a maximum packing efficiency of about 60%-65% by volume, with 35%-40% void volume. This provides an efficiently continuous volume for electrolyte, which is usually added as a liquid solution or solution-swollen polymer which can conform to the void volume space.

A starting rule of thumb for volume of conductor (such as roughly spherical graphite particles) is that 20%-25% by volume is needed to have an efficient conductive network contacting all or most of the active material particles. However, this volume requirement can be reduced dramatically with selection of shape and size of the conductor particles relative to the active material shape and size. High- or low-aspect ratio particles (needles or platelets) are much more volume-efficient in establishing the needed electrical network of contacts to the active material particles. Conductor particles much smaller than the active material particles can occupy void spaces between random-packed active material particles, whereas larger conductor particles similar in size to the active material particles will add to the random packed solids volume, lowering the active material volume to less than the optimal 60%-65%. The high- and low-aspect ratio particles require lower volumes for an efficient conductivity network, but can present challenges for high-speed electrode assembly processing. Another volume-efficient approach is to coat each active material particle with a very thin conductive coating that adds very little to the particle size and its packing.

To function usefully, an electrode must have both electronic and ionic communication to the site of each electrochemical half-reaction. This means that for high efficiency utilization of active materials, attention must be paid to the electrical and ionic conductivity networks within the electrode structure, so that each particle of active material takes part in the reaction.

Electrolyte adsorption on active particle surfaces and volume changes occurring  during discharge or charge will in most cases destroy the integrity of the electrode structure turning it into mush unless a binder is present. Since binders are solids, they will take volume away from the other electrode components. Binder particle shapes affect their volumetric efficiencies, also. When PTFE is used, much lower volumes of binder can suffice if the PTFE is fibrillated to form long, very thin strands. Soluble binders operate as “sticky” coatings holding particles together.

The volume efficiency of high aspect ratio particles is seen also with some forms of acetylene black in which nanoparticles link themselves into strands that provide a highly volume-efficient conductivity matrix.

3 – The Role of Active Material Properties

So far, we have addressed spatial considerations independent of the properties of the active materials themselves. Just as the electrical conductivity network and the ionic electrolyte networks are critical to efficient electrode structures, the electrical and ionic conductivities of the active material particles themselves are important to the efficient utilization of each active material particle. With the electron transfer reaction taking place typically at an active site on the surface of each individual particle, the electronic and ionic conductivities of the particles influence electrode design, also. If either the electrical or the ionic conductivity of the active material is low, then use of finer particles can make a big difference in the power capability of the electrode. Reducing the physical distance electrons and/or ions must travel within the particle to assure high-efficiency utilization is accomplished with finer particles. Material and process costs are generally lower the larger the particle size, so selection of active material particle sizes usually represents a balance between cost and the desired rate (power) capability for the intended application(s).

A typical electrode composition will include active material, electronic conductor to distribute and collect current from individual active material particles, electrolyte to allow the ion transfer necessary to and from the active material particles to support the electrochemical reaction, and often a binder to preserve the electrode structure during the volume changes that occur with charge or discharge.

To summarize

  • Engineering of relative volumes of electrode components must take into account both charged and discharged states of the electrode.

  • The electrode volume devoted to conductor material must be adequate to provide low-resistance electrical contact between the collector and all active material particles.

  • Particle size and aspect ratio relative to active material particles have a strong effect on the volume percentage of conductor needed.

  • The electrode volume devoted to electrolyte must be adequate to provide low-resistance ionic continuity between the separator and all active material particles. 

As always, talk to us if you have battery related questions, or need help with deciding on whether application specific battery development is right for you. You can also visit our battery resources page or battery fundamentals page for more basic information that you may find helpful when doing your background research.

About the Author

John

Hi, I'm John, editor-in-chief of an Flexel Battery online magazine!

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