Explanation of Complex Battery Multiphysics Using Common Day-to-Day Phenomena

This story is contributed by Anirban Roy, PhD

• Battery multiphysics, such as overpotential, pseudocapacitance, phase change, and surface functionalization, are difficult concepts for laymen to interpret
• These concepts are easier understood if related to common day-to-day activities

Figure 1: Variation of multiple forms of overpotential with current.

Motivation

Last year, I took a break out of my professional life to work on a career switch to the energy storage domain. I quickly noticed that there were several complex physical phenomena, which were better understood if we found an analogue to regular events from our day-to-day lives. This article explains four battery physical parameters , overpotential, pseudocapacitance, phase change and surface functionalization, using analogies from daily life.

Overpotential

The overpotential, regularly denoted by the Greek letter eta (ɳ), is defined as the excess voltage over the thermodynamically equilibrated potential, typically referred to as open circuit potential (OCP) for a single electrode or open circuit voltage (OCV) for a cell. It is a representation of multiple irreversibilities that occur inside the cell. Upon cell operation (charge/discharge), these irreversibilities begin with activation polarization, where a fraction of the open circuit potential is utilized to overcome the activation energy of the charge transfer reaction in order to get the current flowing. Next, as the current is flowing, it faces Ohmic resistance. At higher current loads, the lithium concentrations at the surface get depleted very quickly, and a steep gradient in concentration is established inside the electrode — a phenomenon known as concentration polarization. All these phenomena lead to the requirement of additional “overhead” potential on top of the open circuit value. The advent of multiple forms of overpotential with current is depicted in Figure 1. The results are from a fuel cell review, but overpotential is commonly encountered in almost every electrochemical technology such as fuel cells, zinc batteries and vanadium flow batteries.

Real-life analogue for overpotential: commutation time

Say that you are headed to a concert that starts at 2 PM.To start, you first need to walk to the parking lot, which takes 10 minutes(t_walk). The drive from your parking lot to the arena (t_drive) is approximately 30 minutes. Next, the road to the concert passes through downtown, where there can be potential traffic delays (t_traffic), so you add 30 minutes on top of 30. In addition, you also consider potential “circling around” to find an appropriate parking for your car (with maybe an appropriate charging facility as well!), which makes for an additional 30 minutes (t_park).

Therefore, although your nominal time of travel is 30 minutes, an additional 70 minute “resistances” or “irreversibilities” (t_walk+t_traffic+t_park) increases the actual time to 100 minutes.

Here,

t = t_drive + t_traffic + t_park ~ total voltage,

t_drive ~ OCP or OCV,

(t_walk + t_traffic + t_park) ~ ɳ.

Additionally,

t_walk ~ activation overpotential

t_traffic ~ Ohmic overpotential

t_park ~ concentration overpotential (unavailability of parking spot ~ unavailability of reactant)

A schematic is depicted in Figure 2, representing the thermodynamic voltage (OCP/OCV) and overpotential. Additionally,

Figure 2: Pictorial representation of overpotential. Here, Δ₁ represents the thermodynamic (or open circuit) potential while Δ₂ represents the actual potential.

Pseudocapacitance

Concept and applications

The original concept of capacitance refers to the electrostatic accumulation of charge on a surface. Some newly-discovered materials exhibit a phenomena called “pseudocapacitance”, described by fast Faradaic reactions and subsequent inter/de-intercalation processes predominantly occurring at the particle surface. Such materials can also aid in applications requiring higher power density, such as Vertical Take-off and Landing (VTOL) in aircraft where even an 80% SOC with standard intercalative material lacks the necessary power density due longer diffusion lengths and has the tendency to cause concentration polarization. Additionally, nanosized materials, such as lithium manganese oxide (LMO) crystals, can help offset some problems associated with bulk particles such as phase change on intercalation and subsequent Cathode Electrolyte Interface (CEI) rupture (described in a separate section in this article).

How does pseudocapacitance differ vis-a-vis standard intercalative electrode processes?

Commonly used cathode materials such as Nickel Manganese Cobalt (NMC) use diffusive intercalation, where Li ions diffuse into the cathode particle lattice structure. These particles are on the order of microns, implying a larger characteristic diffusion time scale, which results in slower energy transfer and lower power density. On the contrary, pseudocapacitance happens in nanosized particles under a critical length scale, allowing for shorter path lengths, quicker charge (and energy) transfer, and faster charging times.Therefore, while most battery electrodes are based on slower intercalation processes, materials with pseudocapacitance allow for phase change on timescales closer to electrical charge transfer as opposed to chemical reactions, as illustrated by examples in the literature.

Figure 3: Schematic of pseudocapacitive process.

Real-life analogue: email response

Pseudocapacitance versus diffusive intercalation can be explained by a very simple phenomenon: response to email. Imagine you write an email to person A, who replies to you after one week, with a long email that takes you 15 minutes to read and comprehend. Contrast that to person B, who replies to your email in an hour, with a crisp response that takes you just a few minutes to sink in. Person A is an example of diffusive intercalation, while person B represents pseudocapacitance.

Phase change in cathode particles

Background and physics

Lithium manganese oxide (LMO), a layered cathode material, boasted of higher storage and more convenient intercalation due to its 3-D structure, as opposed to 2-D structures such as Nickel Manganese Oxide and 1-D options such as LiFePO4. Additionally, it provided a comparably safer option vis-a-vis existing chemistries due to lower chances of thermal runaway. However, it ran into trouble due to poor cycle life. The problem was attributed to cathode rupture, leading to manganese leaching into the electrolyte and poisoning the graphite anode.

Cathode rupture is caused by an intra-particle phase change. Consider a discharge process when lithium atoms are being added into the particle. As the lithium concentration inside the particle increases, it begins to condense out into two separate phases — one where the proportion of Lithium is high (Li-rich) and another where the proportion is low (Li-poor). The repulsion between two partially miscible phases (similar to oil and water) causes the particle to swell and crack. This in turn ruptures the cathode electrolyte interface (formed by reaction between electrolyte solvent and lithium ions), and releases manganese ions into solution.

Real-life analogue: salt evaporation

Consider the solution of common salt (NaCl) in water. As we heat this solution, water evaporates, and crystals of salt begin to appear. Therefore, two separate phases start showing up.

Here,

water ~ cathode,

salt ~ lithium atoms/ions,

salt crystals ~ Li-rich phase

remaining salt solution ~ Li-poor phase.

Figure 4: Phase change processes inside cathode particle.

Surface functionalization

Interface layers, such as the Solid Electrolyte Interface (SEI) commonly encountered in anodes and cathode electrolyte interface (CEI) for cathodes, are formed due to reaction of lithium ions in the solution with electrolyte solvent. The properties of this interface are influenced by many factors, which include the rate of charge and electrolyte composition. Additionally, the initial layer formed is helpful to passivate the electrode surface from further corrosion, but as it begins to build with battery usage,, it adds to a resistance in charge transfer. It also tends to rupture (again depends upon composition), which in turn creates problems such as leaching of manganese as described in the previous section.

Real-life analogue: body lotion

One solution to the above mentioned problems is to permanently functionalize the surface, thereby minimizing the propensity of SEI rupture. This can be analogized to the usage of cream/body lotion in winter, which functionalizes the surface of the skin and prevents dry skin and cracks.

Here,

cold cream/body lotion ~ functionalizing agent

skin ~ electrode surface

dry skin cracking ~ SEI rupture.

Dr. Anirban Roy is a freelancer with MOEV, a smart charging startup, and Nanodian, a startup focused on developing nanosized LMO cathode particles. He helps both companies with their business development work and also working on developing a model to understand pseudocapacitive charging and discharging. Dr. Roy holds a PhD in Mechanical Engineering from Carnegie Mellon University. In his free time, he is an automotive enthusiast.

Interested in publishing in Battery Bits? Apply at this link to become a contributor.