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Mass adoption of EVs in India has been hindered for many reasons, with one of the major ones being the inability to quick-charge a vehicle to a full range Graphite Rod Electrode
There are certain attributes that must be addressed to ensure EV fleets run on optimal performance
As and when the current bottlenecks are resolved, fast charging is bound to play a pivotal role to bring down range anxiety among users
Mass adoption of EVs in India has been hindered for many reasons, with one of the major ones being the inability to quick-charge a vehicle to full range like that of ICE vehicles.
A lot of engineering efforts have already been put into the current EV platforms that can offer over 80% charge with fast charging. Various EVs from OEMs like Hyundai, Audi, Kia, and more are now able to offer more than 150 Km range with only about a 10-minute charge.
Since EV batteries are prone to accelerated degradation of capacity and cycle life, it would take significant efforts for today’s battery engineers to develop capabilities that offer a maximum range with minimum charging.
Further, while fast charging brings less burden on the overall infrastructure of large EV fleets, but then certain attributes must be addressed to ensure their optimal performance. These factors include:
Operating temperature plays a critical role in designing EVs with fast-charging capability.
At low temperatures, the movement of Li-ions is slower in electrodes and electrolytes. This results in performance drop and ability to fast-charge, which increases the risks of Lithium plating and dendrite formation at the Graphite anode (which operates very close to Li/Li+ potential).
This, in turn, impacts capacity retention in long run, and may also result in a short circuit if Li-dendrites break the separator and establish electrical contact with the cathode to cause short-circuiting of the cell.
A higher temperature promotes faster kinetics and hence significantly subsides the chances of short-circuiting during fast-charging operations. In principle, however, an extreme temperature, typically above 45 o C, must be avoided for battery operations. This is because it might lead to gas release and other several side reactions.
Hence, defining a threshold temperature is key and this depends on many factors, including the cell parameters, age and C-rates.
Fast charging generates heat, which is difficult to remove in an effective and homogeneous manner. This impacts cell-to-cell behaviour and performance deterioration leading to safety concerns.
Failure to contain that heat can lead to uncontrolled temperature rise which can impact the cell and battery pack life and can potentially trigger a thermal runaway event causing battery fires.
Due to the high current during fast-charging operation, the chances of overvoltage increase significantly and go beyond the safer recommended values. This might, at times, cause cells to burst and catch fire.
In fact, overvoltage was one of the major reasons for many EV fire incidents in India, earlier this year. It is also necessary to have a highly advanced and extremely sensitive Battery Management System (BMS) for packs. BMS helps predict and cut off the current before an overvoltage phenomenon is underway.
Graphite is widely used as anode material in batteries today, but it’s susceptible to electrode expansion during charge and discharge, Li plating and dendrite formation, as its working potential is very close to the Li/Li + formation.
These issues with graphite anodes exacerbate during fast charging, especially at higher State-of-Charge (SoC). Hence, conventional Li-ion cells are charged slowly and can only be charged at a relatively higher current within a narrow voltage window.
Moreover, the Solid Electrolyte Interface (SEI) on graphite further reduces the kinetics of Li-ions’ migration and makes fast charging unviable without compromising safety and cycle life.
There are other alternatives, such as LTO (Lithium-Titanium oxide), which is used for extreme charging since the issues of Lithium plating and dendrite formation are non-existent in LTO anodes.
Further, there is expansion and contraction in LTO during charging and discharging, which enables fast charging for more than 10,000 cycles without impacting the longevity of the cell. Silicon has also come to the fore in recent times due to its capacity to hold a large amount of Lithium within its structure.
Extensive research is currently ongoing with silicon-graphite composite and high silicon anodes. The high capacity of silicon allows for a significant reduction of anode thickness in cells, which offers high energy density and the ability to fast-charge. However, the industry is still struggling to accommodate a large expansion of silicon during charge-discharge, which considerably reduces the cycle life of EV cells.
The impact of fast charging an EV varies with cell format selection and design. Since heat is generated during fast charging, larger format cells are more susceptible to its negative impacts. High current densities, close to the tabs, impact cell performance due to non-homogeneous charge distribution leading to poor cycle life.
The tabless design, where electrode foil itself is used as a tab for current collection, is off late gaining traction for larger format cells, enabling fast charging. With uniform current distribution, there’s even heat dissipation with no hotspots generated that could potentially impact safety and cycle life.
Tabless cylindrical format eg. 4680, commercialised by Tesla, is becoming a go-to form factor to enable fast charging. Across different form factors, heat dissipation across the neighbouring cells is more distributed in cylindrical as compared to other formats.
Electrode thickness also plays a crucial role herein; usually, a thicker electrode cell is designed to meet high energy density needs, but suffers during fast charging due to diffusion barriers, creating a concentration gradient.
Additionally, fast charging induces severe current inhomogeneity across electrode layers, and anode expansion and results in the delamination of cells without tight external constraints.
Therefore, it is prudent and advisable for cell manufacturers to take a trade-off between energy density and fast charging capabilities based on demand.
Cell-to-cell performance variations could be significantly impacted due to the capabilities and processes in the cell manufacturing process.
Materials handling and their shelf life, effective slitting and winding of cathode, separator and anode, right placement of the tabs along with proper welding are certain areas that may significantly impact the durability of a battery pack when exposed to fast charging. Hence, high-quality operations shall help in maintaining low cell-to-cell variability.
To sum up, all the above parameters are critical for designing fast-charging battery packs.
New battery materials and advanced engineering efforts have shown vast improvement in fast charging characteristics, compared to current technologies which significantly compromise battery life.
Pure Graphite Sheet As and when the current bottlenecks are resolved, fast charging is bound to play a pivotal role to bring down range anxiety amongst users as well as has the potential to down-size batteries and the associated costs.