UHPC 101 Part 1 – Coulombic Efficiency

Introduction

The first battery was developed in 1800, and the first fleet of electric taxis wasn’t made up of Teslas, nor were they yellow, because in 1900 Tesla didn’t exist and taxis weren’t yellow. Since 1800 immense efforts and resources have been dedicated to understanding and developing better batteries. Much has been learned, but above all, humans have learned just how complicated batteries really are.

From electrolyte blends to voltage and temperature ranges, to sizes and shapes of micron-scale electrode particles, optimizations are required for every battery chemistry and it seems there are always exceptions to the rules and very few principles are universally applicable to all battery types.

While appreciation for the complexities of a system is key when deciding how to approach a challenge, a wise person once said that “perfect” isn’t when you can’t add anything more to an idea or design, rather, “perfect” is when you can’t take anything else away from an idea or design without fundamentally changing it. Simplicity is powerful, and it can be perfect.

To NOVONIX, the simplest and most powerful battery concept of all is Coulombic Efficiency: “How much of what is put in, is returned back out?”. After all, that’s the job of a battery; to store energy so that it can be used to get work done at a later time and place, as is needed. It then seems obvious; the more a battery can give back of what is put in, the better that battery should be. This is Coulombic Efficiency.

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How much water is left…

After 10 Cycles?

After 100 Cycles?

After 1000 Cycles?

In this series of blog posts, we dive into the key fundamental metrics that we regularly rely on to describe battery system performance. In exploring these fundamental metrics, you’ll learn that:

  1. Going slow can teach you more, faster,

  2. Batteries can be complicated, but measurements don’t need to be,

  3. Good measurements lead to good decisions,

  4. Understanding reactions at electrodes can guide path to market.

So let’s start by discussing Coulombic Efficiency. And keep your eyes peeled for more posts on the UHPC 101 series.

What is Coulombic Efficiency?

Coulombic Efficiency (CE) is the single most important metric when projecting the future performance of a battery. A CE of 1.00000 means a battery would last forever. A CE change on the order of 0.0001 could correspond to a difference in cycle life of a thousand cycles or more.

Unwanted reactions in a Li-ion cell lead to loss of reversible capacity over time. Ultra-High Precision Coulometry (UHPC) measures how much reversible capacity is lost cycle-to-cycle, even in very small amounts. The discharge capacity to charge capacity ratio gives the fractional reversible capacity of a cell and is called the Coulombic Efficiency (CE).

Careful analysis of Coulombic Efficiency, if measured with sufficient precision and accuracy, can provide insight into degradation mechanisms within a cell and allows quantitative comparisons between cells after only a fraction of their cycle life.

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Isolating degradation mechanisms requires slow cycling of cells (e.g. C/20) which can seem counter-productive. However, the differences in CE after only 20 days forecast long-term differences in capacity retention otherwise invisible.

In the second post of the UHPC 101 series, we explore how to compare the CE of cells cycled differently, whether at different rates (currents) or to different states of charge (voltages). It is important to understand the time dependence of CE – as explained above, time-dependent degradation reactions within the cell will lead to higher CE values for a cell cycled faster, compared to the same cell cycled slower. To expand the usefulness of CE and to make the best decision for your product or technology application, CE must be normalized for time, broadening the scope of applicability. This is Coulombic Inefficiency per hour (CIE/hr).

In its most basic form, CE is made-up of two ingredients: capacity fade and charge end-point capacity slippage. These and other UHPC metrics and techniques will be described in our next posts.

Check our blog page regularly to keep up to date on the UHPC 101 series!

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