The Problem With Fast Charging

One of the major drawbacks of EV over the IC engine vehicle is charging time. As we know filling the gasolene tank only takes a few minutes. however, for the same drive range, it takes about an hour for an EV to recharge itself even with a fast charger. In this blog, I will discuss some of the problems with fast charging in the case of LiB. Before we dive into what happens inside LiB when we try to fast charge, I would like to talk a bit about LiB construction. 

LiB Construction

In my previous blog, 'How Does The LiB Work?', I indicated LiB as a simple construction with 2 electrodes (Anode and Cathode) placed in an electrolyte. As shown in figure 1.

Figure 1. Oversimplified LiB

However, the construction of an actual Lithium-ion cell is not at all like what I have shown above! In an actual cell, Anode and Cathode are placed very close to each other in order to pack as much active material (material which takes part in intercalation/de-intercalation) as possible in a given volume. An actual Lithium-ion cell looks something like figure 2

Figure 2: Actual battery construction. Cross-section microscopic image adapted from Schmidt, Ute & Ayasse, Philippe & Hollricher, Olaf. (2016). New Developments in rising Microscopy: Correlative Raman and SEM Imaging.

As can be seen in figure 2, the anode and cathode are very closely packed. In order to avoid direct electrical contact between anode and cathode, a separator is used. A separator is soaked in the electrolyte (in case of liquid electrolyte). The functioning of some of the basic Li-ion cell components is as below:

• Anode/Cathode conductor: These are very thin (few microns) metallic sheets (generally Aluminum for the cathode and copper for the anode). They conduct electrons to-and-fro active materials coated on them.
• Separator: This a very thin sheet of polymer. Its main function is to allow movement of Li+ ions while disabling electrical contact in between anode and cathode
• Electrolyte: it enables the movement of Li+ ions to-and-fro anode and cathode. LiB Electrolyte contains Lithium salts like LiF, LiPF6, etc dissolved into an organic solvent. In most of the cases, an electrolyte is highly flammable and volatile in the open air. A lot of research is concentrating on how to make electrolyte safer. In solid-state LiB, a solid electrolyte is used in place of liquid electrolyte. this makes solid-state LiB safer.

Fast Charging

Fast chargers can charge EV battery typically in about 1 hour up to 70-80%. However, if we look at the following graph, the battery degrades faster if we use fast-charging (this may not be 100% true for all the advanced EV batteries where special care is taken to avoid accelerated degradation in case of fast charging)

Figure 3. The capacity degradation upon continuous cycle aging at different charging current rates (ref: Yang Gao, Jiuchun Jiang, Caiping Zhang, Weige Zhang, Zeyu Ma, Yan Jiang, Lithium-ion battery aging mechanisms and life model under different charging stresses, Journal of Power Sources, Volume 356, 2017)

*Charging rate: to explain in very simple words, for 100Ah battery,  1C charging current refers to 100A.

Let's see what exactly happens when we charge a LiB. For a better understanding of LiB operation, you may have a quick look at one of my previous blogs 'How Does The LiB Work?'. Following video shows what exactly happens when we charge a LiB:

I recorded this video at Battery Japan Show. It is an actual cross-sectional microscope footage of battery charging-discharging in real-time.

Blue Graph indicates the voltage of a LiB and the red line indicates charge-discharge current. The upper part of LiB is Anode, the middle white part is separator + electrolyte and the lower part is the cathode. At the time of charging, Lithium-ion intercalates into the anode. thus we can see a change in the color of the anode from black (C- graphite) to golden (LiC6). Charging and discharging process can be described as shown below:

Figure 4. Reactions at the Anode

Intercalation of Lithium-ion into the anode also expands anode in case you have missed in the video. However, when the charging current increases, all the Lithium-ions cannot intercalate into the anode as shown in this video:

I recorded this video at Battery Japan Show. It is an actual cross-sectional microscope footage of battery charging-discharging in real-time.

As you can see, some of the Lithium-ion fail to intercalate into anode material and form a layer of metallic lithium on the surface of the anode (silver color). If we keep on repeating this fast Charging process, some of the lithium-ion permanently form a metallic lithium layer on the anode (they cannot go back into the cathode). Some times this layer becomes so much thick at places that it penetrates the separator and internal anode-cathode shortcircuit happens (also know as dendrite formation).  This results in catastrophic results.

There is no easy way to avoid this phenomenon. some of the solutions may involve increasing strength of separator (by increasing thickness or by coating separator with other materials) so that lithium metal dendrites will not be able to puncture the separator. However, this results in increased resistance of separator for the Li-ion movement.

Apart from the above-mentioned mechanics, there are many other things happening in parallel in case of fast charging which also contributes to the battery capacity fade over time. For example, mechanical stresses due to the rapid expansion of anode, thermal effects, etc. So in the nutshell, it is not easy to optimize LiB for fast Charging!

I hope after going through this blog post, you will appreciate engineering efforts when you simply plug-in your EV to a fast charger :D

Keep exploring!

-- Harshad