Discussing technology in the context of the rapid development of lithium-ion power batteries seems somewhat "out of place." Everyone is expanding production capacity, focusing on speed and efficiency. While there may be a sense of awe and apprehension towards technology, this is insignificant in the context of a frenzied market. However, I still believe that only by looking at past technological changes historically—why they changed, how they changed—can we better reflect on whether our initial intentions remain unchanged. Let's just treat this as a story.
Let's begin by introducing our topic with the following four charts—power battery technology has evolved and changed amidst debate. These charts compare the installed capacity of different battery systems, the range of mainstream vehicle battery capacities, and the differences in battery shapes among the mainstream battery manufacturers used by automakers. Each debate has been intense, involving not only the drafters and setters of standards but also the stakeholders (battery companies). Furthermore, each resulting change and adjustment has been painful and often iterative.
Since the beginning of research on power batteries, the following four aspects have been the subject of constant discussion and debate.
First, the debate over material systems.
This has been discussed extensively, especially regarding ternary lithium batteries and lithium iron phosphate batteries (see: The debate between ternary and lithium iron phosphate batteries is necessary and does not negate the value of ternary materials). To put it dialectically, there are no inherently bad materials; they are simply not suitable for optimal application. Actual data shows that lithium iron phosphate is more suitable for buses, while ternary lithium batteries have a clear advantage in passenger vehicles (the decline in ternary lithium battery market share in passenger vehicles from January to April 2016 was due to the "temporary suspension" policy). The remaining issue is how to mitigate and resolve safety concerns from multiple dimensions, including materials, battery design, and pack design. Optimistically, the temporary suspension of ternary lithium batteries is technically temporary, and technological improvements can compensate for their safety shortcomings.
In addition, for the negative electrode side, lithium titanate batteries (the negative electrode uses lithium titanate-LTO, not graphite) have a place in certain specific fields due to the safety and fast charging advantages brought by zero strain (fast charging vehicles, energy storage); hard carbon batteries, due to their superior ability to insert and extract Li compared to graphite (the lattice interlayer spacing is larger than that of graphite materials), are also gradually being paired with ternary batteries for use in hybrid vehicles and other fields.
Second, the debate over capacity.
In the early stages of power battery development, it seemed that the larger the capacity, the better. Some companies claimed to produce 400Ah or 500Ah batteries, which generated considerable buzz and was widely publicized. However, this trend eventually faded into obscurity. From the perspective of current process control and technology, this approach is unscientific and unreliable, merely a marketing gimmick. Who would dare use such batteries? A reasonable capacity threshold should be a value based on a comprehensive balance of material systems, size design, and process technology. The table below lists some typical capacities from battery manufacturers. Furthermore, is a small capacity necessarily better? I recall an industry standards discussion meeting in 2009 (I won't mention specific standards to avoid controversy), where the capacity range for electric vehicle batteries was discussed. One company executive confidently stated that a capacity greater than 5Ah would suffice (someone suggested defining it based on energy conversion function, i.e., converting chemical energy into kinetic energy, regardless of size, all should be classified as power batteries, but this was rejected). I wonder what he would think if he saw the mass application of 18650 batteries today.
JBT11137-2001 recommended some capacities, but due to the special products of battery manufacturers at that time, they are not generally used now, and there is no universally accepted standard to define the capacity range.
Then everyone can use their own creativity, but the scope for creativity will be gradually defined for you – that's the limit!
Third, the size dispute
Like capacity, battery size is a highly debated topic. The same capacity with different dimensions results in different performance characteristics (especially rate capability and lifespan). The image below shows three different sizes of a 20Ah battery (thickness not indicated), clearly demonstrating significant differences in power characteristics. Currently, although there is a recommended standard in China: QCT840-2010 (Specifications and Dimensions of Power Batteries for Electric Vehicles), it is not widely adopted due to insufficient representativeness or other considerations for improvement. Europe has VDADIN-91252, which German automakers require to supply batteries according to this standard, and many domestic battery manufacturers are also developing batteries according to its size requirements. A major reason why domestic standards are not widely adopted is the lack of participation from vehicle manufacturers, and the absence of unified size requirements among them.
Currently, the VDA standard targets passenger cars. With a given size, the capacity is left to its own devices, and the extent of the flexibility depends on each manufacturer's capabilities. In the bus sector, however, the situation is still fragmented, with a wide variety of sizes leading to diverse capacity products. Standardization will take time and effort.
Fourth, the debate over shape.
Which is more suitable: wound, rigid-shell prismatic, or soft-pack? Wound small cells such as 18650 and 26650 are already widely used as standard products, featuring mature automated production equipment, stable processes, and good consistency, achieving defect rate control at the ppm or even ppb level. Rigid-shell prismatic cells have advantages such as high dimensional accuracy and strong voltage resistance. Soft-pack batteries have advantages such as good safety control and high degree of flexibility in design. However, the choice of battery shape requires consideration not only from the perspective of the battery itself but also from the comprehensive consideration of module design and assembly technology; it cannot be generalized.
Amidst the debates, lithium-ion power battery technology has overcome its period of uncertainty. In this process, China has also made continuous progress, from learning and modifying as it goes, to finally finalizing its design.
Currently, in these four aspects, the materials system is basically divided into two groups; volume and size are gradually moving from disorder to order; and shape is divided into three groups. Things have become much simpler and more orderly. The rest is about exploring these rules further and further, and improving as we go.
