Li ion batteries (LIBs) are emerging as an important part of in our day-to-day lives via portable electronic devices such as mobile phones, laptops, electric vehicles, residential and commercial battery energy storage devices, power tools and so on. From smart phone users (more apps and longer operation time) to electric vehicle owners (range anxiety-free), the predominant preferred features of LIB are energy density and life span. Nowadays, the fast-charging feature of LIBs is gaining a lot of interest among tech savvy users to see how their devices can be charged within minutes than in hours. This is particularly important for Electric vehicles users where some prospective buyers remain hesitant due slow charging speeds currently. While drivers today are accustomed to filling up in less than 5 minutes, EVs, depending on the size and specifications of the battery, typically take at least 30 minutes to get 80 percent charged at the fastest charging stations out there.
A charging cycle is when a battery goes from being fully charged to empty and then from empty to fully charged. Let’s understand this charging-discharging feature, of a LIB. During the charging process positively charged lithium ions are extracted from the cathode (positive electrode) move through the electrolyte and the separator and are inserted into anode (negative electrode) material. During the reverse discharging process lithium ions are extracted from the anode and inserted back into the cathode, the electrons are transferred via an external circuit and the stored electrical energy is utilized to power mobile phone, electric vehicles (EV), power tools etc.
The capacity (the total charge stored) of a battery is measured as Current × Time in Ampere-hours (Ah) when charged and discharged at specific currents.
C-rate of a LIB is a measure of the rate (power) at which a battery is fully charged and/or discharged to its rated capacity. Generally, the C-rate is specified in terms of the charging/discharging currents normalised against the rated capacity of a battery.
For example, 1C rate of a 10Ah battery equates to a charge and discharge current density of 10A. In other words, at 1C charge and discharge rate (charging and discharging currents of 10A) the battery will be fully charged and discharged in 1 hour.
At C/2 rate, the same battery will be fully charged and discharged at currents of 5A in two hours. A higher 2C rate, corresponds to charging and discharging current of 20A where the battery will be expected to be fully charged and discharged in 30 mins. Commonly used battery C-rating is as follows:
| C-rating | Battery Charge/Discharge time |
|---|---|
| C/20 | 20 Hours |
| C/10 | 10 hours |
| C/5 | 5 hours |
| C/2 | 2 hours |
| 1C | 1 hour |
| 2C | 30 minutes |
| 4C | 15 minutes |
| 5C | 12 minutes |
| 10C | 6 minutes |
| 20C | 3 minutes |
Fast charging requires high current densities to charge a battery to its rated capacity. At higher C-rates, batteries face high internal resistance during the extraction and insertion of lithium ions from cathodes and anodes and during the transport through the electrolyte. Furthermore, at higher C-rates, the increased internal resistance can overheat the battery, decompose the electrolyte and eventually affect the performance and the life span of the battery.
The LIB chemistries employed in commercial batteries of current EVs can retain 80% of their initial capacity after 1000 cycles at lower charging currents. At fast charging rates these LIB’s have reported to show drastically reduced cycle life.
Magnis’ technology partner, C4V has developed a cutting-edge proprietary LIB technology called Bio-Mineralised Lithium Mixed-Metal Phosphate (BMLMP) which supports the fast-charging capability. In addition to enhanced safety, longer life span and lower cost, BMLMP technology with its fast-charging feature has been commercialised and used in live applications in LIB cells that are currently being produced at iM3NY’s LIB Gigafactory.
