India has set a very audacious target of converting 30% of its overall vehicles to electric by 2030. The efforts to meet this target are pretty much visible with the recent development in government policies like FAME-2 announcements as well as the PLI scheme for battery manufacturing in India announcement.
As the interest for Lithium Batteries increases, the worry and willingness to charge the vehicle at a faster rate also increases to overcome the range anxiety. I am often asked about the ways to charge the battery at a faster rate so that refilling can be done in an hour or so. A lot of efforts are being put by the scientific community to develop the fast-charging batteries by changing the chemistry of the cell either by changing the cathode, or changing the anode, or developing the all-solid-state batteries.
However, this article is about discussing the fast charging of very popular and existing chemistries such as NMC and LFP batteries.
How fast we can actually charge these chemistries and why it is not possible to charge it above certain charge rate and what are the effects:
Before understanding the faster charging, we need to understand the Charge Rate generally called the “C” rate in the battery community to make the explanation simple.
So, C rate is simply an index that helps a battery engineer to tell you about the charging and discharging current.
The basic representation of the C rate is done by writing the letter “C” followed by a Number or sometimes a number followed by the letter “C” like C2 or 2C respectively.
So, if you have knowledge of the C rate and the Capacity of the battery, you can calculate the Current and Time to Charge or Discharge the Battery fully.
Let say you have a battery of capacity 3Ah and you are charging it at 3 amp then the C rate is simply a division of 3Ah by 3A and the C rate is 1C. So with 1C rate, you can charge or discharge the battery fully in one hour and with 2C you can charge or discharge the battery in ( ½)half an hour.
Similarly, for 3C you can charge or discharge the battery in (1/3) Hour.
If you carefully see the cell specification sheet, LFP batteries are usually rated at 1C and capable of charging and discharging at 1C and NMC cells can be charged and discharged at 3C rate. Why the two battery chemistries are rated at different C rate?
Let’s understand this!!!
When you put current inside the cell, the lithium moves from one side (Cathode) to other side (Anode) and it gets stored on Anode side, this is called charging. Now when you put a load to use the battery, it is discharging and again the lithium atoms move from Anode side to the first one (Cathode side). The lithium reached at other side further goes inside the cathode or anode depending upon the charging and discharging sequences.
When the lithium atoms move from one location to another, various kind of resistance are involved.
- Activation resistance
- Ohmic resistance
- Concentration resistance
These resistances are inherently coupled with the Material properties of anode, cathode and electrolyte materials inside the cell and that is the reason we see different C rate capability of LFP and NMC chemistries. The cathode material in case of NMC and LFP is different.
The rate of current decides that how fast the lithium atoms can move from one side to the other (Point A to Point B) but the chemistry in combination with current decides how fast the lithium atom moves inside the cathode or anode (Point B to point C).
So, there should be proper balance between these two transports to operate the cell at safe and optimised level.
Now when the lithium atoms are not able transport inside the cathode or anode (Point B to point C) and you are putting lot of current, the lithium instead of going inside the anode and it starts depositing on the anode surface and it grows in form of dendrites, which can give rise to short circuit and failure of the cell over the period of time.
The excess current also give rise to parasitic reaction (solid electrolyte interface growth) and contributes to electrolyte disintegration and loss of electrolyte, due to which the battery fades over time and excessive current charging or discharging is not recommended.
The high current contributes to resistive heating for cell and cell terminals as well as the welding or connection spots. High temperature inside the battery pack again gives rise to temperature inhomogeneity and cell to cell level performance imbalances which further degrades the life of battery pack.High current makes the whole battery pack vulnerable to thermal runaway and safety issues.
These are the reason; you should never try to charge and discharge the battery pack at the current level above the recommended one.
So, what are the approaches to meet the need for Fast Charging:
• Changing the inherent battery material chemistry
• Proper cooling mechanism to overcome the resistive heating phenomena
• Tab and joints should be designed in such a way that the resistive heating is minimum and whatever heat is generated it is dissipated immediately?