Electric vehicle (EV) battery technology has substantially evolved over many decades, largely driven by a certain degree of deficiency in one battery technology that subsequently incentivised the development and deployment of newer battery chemistries aimed at overcoming those deficiencies
The strength of batteries is measured on parameters such as energy density, faster charging, number of duty cycles and wide operating temperature range. The operating environment dictates which battery chemistry will gain prominence in a certain region or country.
ADVENT OF LEAD ACID & NIMH TECHNOLOGIES
Lead acid was the first battery technology to be deployed in battery electric vehicles (BEVs) in the early 90s. These batteries lost out on popularity owing to deficiencies such as lower energy density and lower life. The lead acid technology has been in the market for a long time and there is limited scope for further optimisation as several threshold values have more or less been achieved, explained Ashim Sharma, Partner & Group Head, Nomura Research Institute.
In the 90s, nickel metal hydride (NiMH) battery technology also arrived in the auto space, but was more widely deployed in hybrid vehicles. One of the critical battery requirements in a hybrid vehicle is longevity due to multiple change and discharge cycles during the course of operation, and this explains why NiMH batteries are deployed in hybrid vehicles. NiMH batteries did not gain market acceptance among BEVs because they have limited discharge current (0.2 C - 0.5 C), limited lifecycle and generate heat during fast charging and discharging. These factors limited the performance of EVs, in terms of acceleration performance and fast charging capabilities that are considered crucial for BEVs. Toyota has been at the forefront of deploying nickel metal hydride (NiMH) batteries in its hybrid vehicles in various models such as Prius and Camry Hybrid, etc.
LITHIUM BATTERY CHEMISTRIES
Lead acid and NiMH battery technologies have had certain deficiencies that necessitated the need for a new battery technology. And such a scenario heralded the arrival of lithium-based batteries in the auto space in the late 90s. Among lithium-based batteries, lithium nickel manganese cobalt oxide (NMC) and lithium iron phosphate (LFP) have emerged as prominent lithium battery chemistries for EVs and their prominence can be attributed to their substantial production scale-up over other chemistries over the past decade.
“NMC and LFP have gained market acceptance because their prices also dropped drastically and these chemistries score high on the reliability front owing to their extensive use. NMC and LFP will remain fundamental battery families for EVs as their cells have witnessed significant cost reduction,” explained Nakul Kukar, Co-Founder & CEO, Cell Propulsion.
NMC is considered a good option for long-range EVs, especially passenger cars, sedans and SUVs, where one can pack in more energy in each cell of the EV battery pack to effectively manage the battery pack size, while delivering long vehicle range. NMC is also considered a good option for the Indian two-wheeler segment that grapples with space constraints to install the battery pack. It can pack in more energy using a lesser number of cells, thus resulting in small compact batteries that can fit easily in two-wheelers.
NMC is said to work well in countries that have colder climates, but can have issues if used in hot weather conditions and will need a complex & extensive cooling system and the range benefit is not worth the effort, especially in the Indian context, explained Kukar.
Among other lithium-based battery chemistries, lithium iron phosphate (LFP) works well for three-wheelers in India as it is cost-effective and more reasonably priced than other chemistries, although it cannot provide a longer range. Further, if three-wheelers desire long range capability they can explore NMC but it will only augment the cost and therefore it won’t make any sense for three-wheelers.
LFP can work well with long-haul buses as the latter have adequate volume and mass margin for heavier battery packs and are built with lower energy density cells that can offer required range. “LFP is a desired solution for buses due to its inherent safety and lower cost even though its energy density is lower since there is enough volume in such vehicles and they can easily carry extra mass – even if you add one ton of extra weight, still the bus body and structure will be able to handle it”, opined Kukar. This chemistry can also be deployed for heavy duty trucks up to 40 tonne GVW.
Another technology that is witnessing steady adoption is lithium titanate oxide (LTO) – its biggest advantage is its high cycle life of 10,000 plus cycles and can operate in extremely high temperature. LTO is also not prone to thermal runaway and can support fast charging in high ambient temperatures. According to Sharma, this chemistry offers high discharge rates, which make it ideal for applications such as hybrid vehicles, forklifts, tractors, mining and defence vehicles. However, its price is on the higher side and energy density is a bit lower.
Lithium nickel cobalt aluminium oxide (NCA) is another battery chemistry used in EVs, but hasn’t gained much prominence. NCA shares similarities with NMC, but is costlier and is also regarded as a less-safer version of NMC.
ROAD BEYOND LITHIUM-BASED BATTERIES
Lithium-based batteries have several attributes that have enabled them to be a market mainstay, but there is a question mark over its long-term sustainability as some of the materials (lithium and cobalt) used in making them are not widely available and are only confined to a few nations globally. There is considerable work happening to develop newer, viable EV battery technologies and among them solid-state batteries are considered the best alternative to lithium batteries. Solid-state batteries offer safety features more than what LFP can provide as well as provides energy density close to what NMC offers, and can ensure good range in hot conditions, Kukar pointed out.
The sodium-ion battery technology is also considered a viable alternative to lithium-based batteries – it is similar to a lithium-ion battery, wherein only lithium compounds are replaced by sodium compounds. “The biggest plus point about this technology is that sodium is abundantly available across the globe unlike scarcely available lithium or cobalt and it can also be extracted from sea water, which thereby ensures adequate supply for all countries with a coastline, remarked Sharma.
Sodium-ion batteries offer several advantages such as low switching cost for manufacturers due to similarities in manufacturing processes/ protocols between them and lithium-based batteries. Further, it offers a lower pack cost due to use of cheaper materials (for example, aluminium is used in current collectors as opposed to copper being used in lithium-based batteries). The sodium-ion technology can ensure fast charging by leveraging a right combination of hard carbon anode and corresponding cathode to eliminate sodium plating as well as ensure easier transportation and storage as compared to lithium-based batteries. However, there are challenges such as development of an effective electrode & electrolyte material and improving the lifecycle that have to be addressed before sodium-ion batteries can be ready for commercialisation, noted Sharma.
It is pretty clear that lithium-based battery chemistries will continue to hold relevance over the next decade or so. But there are reservations over long-term sustainability of lithium-based batteries owing to limited availability of lithium and cobalt as well as its vulnerability to warmer weather conditions. Various technologies such as aluminium air, zinc air, etc are in development stage, but only two technologies – solid state and sodium-ion battery technologies – appear closer to commercialisation. Of course, in an ever-evolving battery technology space, newer possibilities can never be ruled out.
TEXT: Suhrid Barua