Cylindrical, Square, Soft Pack, The Gap Between Electric Vehicle Batteries Is Bigger Than You Think?

We usually like to refer to "battery, electric drive, electric control" as the "three electric" system of new energy vehicles. Through the "strong cooperation" between the three, an electric car will eventually run and become a A pure electric vehicle that achieves mobility. In a simple sense, the so-called "three powers" are nothing more than electric motors, batteries, and electronic control systems that allow the two to "live together".

In order to give you a deeper understanding of the characteristics and connections between the three, the chief travel officer will conduct an in-depth analysis of the "three electric" system of electric vehicles in the form of serial illustrations to help you use the most Bai's way to understand the most essential principles of electric vehicles in the new energy era.

Take everyone first to understand that one of the core hardware of electric vehicles-battery.

What are the "secrets" about batteries?

There are two types of electric vehicle batteries known at this stage, which are divided into ternary lithium batteries and lithium iron phosphate batteries according to the different materials of the positive electrode. The former is currently the most mainstream battery type, and the lithium iron phosphate battery is the "iron battery" that once made BYD famous. It's just that the lithium iron phosphate battery has poor activity, resulting in its low energy density and can't provide longer endurance, so it gradually faded out of sight.

The current mainstream ternary lithium battery has the advantages of high battery activity and higher energy density, so new energy vehicles basically use ternary lithium batteries as energy storage mechanisms. The ternary lithium battery is also divided into two categories, one is the MCM (nickel cobalt manganese) ternary lithium battery used by most car companies, and the other is the NCA (nickel cobalt aluminum) used by Tesla Yuan lithium battery.

Regardless of the kind of lithium battery, its essential structure is similar. All are composed of positive electrode, negative electrode, separator and electrolyte. Lithium battery charging is to generate charged lithium ions (equal amount) from the positive electrode, and detach from the positive electrode, "swim" the electrolyte and the separator to the negative electrode, and insert it into the negative electrode material. The discharge process is the opposite. Lithium ions escape from the negative electrode and "swim toward" the positive electrode. In simple terms, the charging and discharging process of lithium batteries is realized by lithium ions “swimming” back and forth between the positive and negative electrodes.

It is electric current that pushes lithium ions back and forth. So we can simply understand fast charging as a high-power thruster behind lithium ion, which quickly and forcibly pushes lithium ions from the positive electrode to "swim" the negative electrode, while slow charging is a small power thruster, with lithium ion slow and slow Swim from the positive to the negative.

So why does fast charging have a certain effect on the battery? Quite simply, many lithium ions with high-power thrusters "crazed wildly" from the positive electrode to the negative electrode, and before reaching the negative electrode (embedded in the negative electrode), another lithium ion at the back also rushed over, and two lithium ions collided Together, they “crashed” and lost their activity. As a result, the battery loses one lithium ion. Over time, the "dead" lithium ions will pile up and form lithium dendrites. Many battery deflagrations are mostly caused by lithium dendrites puncturing the separator too long and causing a short circuit inside the battery.

So why does fast charging have a certain effect on the battery? Quite simply, many lithium ions with high-power thrusters "crazed wildly" from the positive electrode to the negative electrode, and before reaching the negative electrode (embedded in the negative electrode), another lithium ion at the back also rushed over, and two lithium ions collided Together, they “crashed” and lost their activity. As a result, the battery loses one lithium ion. Over time, the "dead" lithium ions will pile up and form lithium dendrites. Many battery deflagrations are mostly caused by lithium dendrites puncturing the separator too long and causing a short circuit inside the battery.

Before we understand the batteries, we must first know that the "battery pack" and "power battery pack" that are often said at present are not a single battery body, but are composed of several batteries (single cells), conductive rows, sampling units and After some necessary structural support components are integrated together to form a module, they can be called "battery packs" or "power battery packs". The battery cell (single battery) itself has different forms, mainly divided into three types: square hard shell battery, cylindrical battery and soft pack battery.

Most new energy car companies love to use: square hard shell battery

The square hard-shell battery is arguably the most widely used battery form. At this stage, in addition to Tesla, more than 90% of new energy vehicles use this battery form. The mainstream domestic battery suppliers represented by the Ningde era also take rectangular hard-shell batteries as their main R & D products. This is also one of the advantages of square hard-shell batteries: there are enough suppliers. For car companies, this also means that the cost of purchasing batteries can be effectively reduced.

In addition, the square hard-shell battery itself has a higher space utilization rate, so the volume and capacity of the battery cell are also significantly better than other battery forms, and the battery energy density can also be made higher. Taking the NCM811 battery of the Ningde era as an example, after the PACK can be achieved, the overall energy density of the battery pack exceeds 180Wh / kg. Simultaneously. Larger cell volume and capacity mean that the number of PACK groups decreases, which also means that the requirements for the BMS battery management system are reduced.

But the disadvantage of the square hard-shell battery is that before the PACK is assembled, the battery itself needs a separate outer hard protective shell, which means that the overall weight of the battery pack has increased significantly. At the same time, higher space utilization also means increased requirements for cooling system layout, which will further increase the design cost of the battery pack.

Even though the current battery housings are beginning to use lighter-weight aluminum materials and smarter cooling designs, these two parts of hardware still exist in essence. Therefore, how to control the overall weight of the battery pack has become the main problem at present.

In order to solve this problem, Ningde Times launched its latest CTP highly integrated power battery development platform, eliminating the battery PACK grouping link and integrating the battery cells directly into the battery pack. Compared with traditional battery packs, the volume utilization rate of CTP battery packs is increased by 15% -20%, the number of battery pack parts is reduced by 40%, and the energy density of battery packs is increased from 180Wh / kg to more than 200Wh / kg, which becomes a square hard shell The best solution for the battery at this stage.

Tesla's "love": cylindrical battery

Cylindrical batteries have always been Tesla's only choice, but Tesla's choice of cylindrical batteries is also a kind of helplessness in a sense. In fact, cylindrical batteries are widely used. As early as 1992, 18650 cylindrical batteries have been widely used in electronic products. 18650 represents the model of the battery, "18" represents the diameter of the battery, "65" represents the height of the battery, and "0" represents the cylindrical battery. Similarly, the 21700 battery used by Tesla is now well understood.

The technical maturity of 18650 battery is very high, and because of its own structural characteristics and standardization, the automation level of cylindrical battery production will be higher. At the same time, major foreign manufacturers such as Samsung and Panasonic can also maintain the yield rate above 98%, and domestic battery manufacturers can basically achieve more than 90%. Therefore, Tesla's choice of 18650 in the initial stage is also a choice for neutralization based on the above reasons.

The advantage of the cylindrical battery itself is that the energy density of the single cell is higher than that of the square hard shell battery. At present, the latest 21700 battery used in the Tesla Model 3 has increased the single cell energy density to 300Wh / kg, which is also another battery form A level that cannot be reached within a certain period of time.

At the same time, the cylindrical battery has excellent cycle performance, can be quickly charged and discharged, has high charging efficiency, and has greater output power. In addition, because the battery technology is more mature, the battery consistency is high, and the overall stability of the battery pack after the PACK is grouped is also better. In addition, because the battery cell energy is small, it is easier to control in the event of a failure. Of course, this has higher requirements for the BMS system.

However, the cylindrical battery itself has a smaller size, which is only slightly larger than the No. 5 battery we use daily, so the 18650 battery itself has a smaller single cell capacity. In order to meet the higher power consumption of electric vehicles, it can only be compensated by increasing the number. For example, the battery pack of Tesla ’s previous models was composed of more than 7,000 18650 batteries, and a more powerful BMS system was needed to control such a large number of batteries. This is one of the reasons why only Teslas have used cylindrical batteries for a long time. .

Secondly, the cylindrical battery itself is a cylindrical body, and the space utilization rate is obviously inferior to that of the square hard-shell battery. But fortunately, a cooling system can be laid in the gap between the cylindrical batteries, which is also a blessing due to disaster.

"Enlarged version" of mobile phone batteries: soft pack batteries

The soft-pack battery can be said to be the least battery form currently used in electric vehicles, but we are not new to it. Most of the batteries in the mobile phones around us are soft-pack batteries.

The biggest difference between the soft pack battery and the other two battery forms is that the shell is made of aluminum-plastic film. Compared to the other two, the battery itself is lighter. At the same capacity, the weight of the soft-pack battery is 20% lighter and the capacity is 50% higher than that of the rectangular hard-shell battery. Therefore, the theoretical energy density of the soft-pack battery is higher than that of the square battery and the cylindrical battery

In addition, another big advantage of soft-pack batteries is that the richness of modular customization is higher, the imagination space in the shape of the battery is larger, and the requirements for placement space and location are lower. This has also prompted many hybrid models to choose the soft pack PACK as a power battery pack.

However, the material of the soft pack battery is a soft aluminum plastic film, and the self-protection of the battery body is poor, so the soft pack battery needs a harder protective case after the PACK group. In addition, the layout of soft-pack batteries is mostly lamination, one piece of soft-pack batteries are stacked vertically, so the layout of the battery thermal management system needs to add a layer of cooling fins between each two batteries. This design not only increases the overall weight of the battery pack, but also has higher requirements for the design layout.

Secondly, the current maturity of the manufacturing process of soft-pack batteries is relatively low, and the main technologies are in the hands of Japanese and Korean battery companies. At the same time, the availability of soft pack batteries also led to a decline in battery production standards and consistency. In addition, pure electric vehicles have lower requirements for the shape of the battery, and the demand for customization is not large, so the soft pack battery has not been able to circulate on a large scale.

More importantly, the production technology of aluminum-plastic film shells required for soft-pack batteries is complicated, and it is basically completely dependent on imports at present. Therefore, the higher purchase cost has also led to the case that domestic electric car manufacturers have basically not selected soft-pack batteries. Of course, except for the future K50.

The future of power batteries still has a long way to go

Although the three battery types have their own advantages and disadvantages, as far as the current new energy market is concerned, battery technology still cannot meet consumers' demand for battery life. Although the cruising range of pure electric vehicles has begun to develop into the "group" of 600km at this stage, the technology of ternary lithium batteries has stepped out of the bottleneck period. At the same time, there are still many deficiencies in the charging speed and the layout of the charging piles.

Therefore, the further development of new energy vehicles, especially electric vehicles, requires not only obvious breakthroughs in battery technology, but also more comprehensive construction of supporting facilities.