From the perspective of Nickel Metal Hydride No. 5 battery, where will the fast charging of new energy vehicles go?

In the use of electric vehicles, consumers are most worried about the charging time and cruising range. Under the current technical level, it is difficult to have both charging time and cruising range. Therefore, power lithium batteries have developed two routes. One is the energy-specific faction that focuses on cruising range. The important thing is to increase the cruising range of electric vehicles by continuously improving the specific energy of lithium-ion batteries; the second is the fast charging faction that focuses on reducing charging time. The important thing is to shorten the charging time of electric vehicles by improving the fast charging performance of lithium-ion batteries. With the advancement of technology and in-depth research on lithium-ion battery materials, the difficulties that fast charging technology has encountered may also be solved one by one.

●Combining opinions from many parties, it can be said that a charging rate less than 1.6C is slow charging, 1.6C-3C is small fast charging, and 3C or above is fast charging.

●Fast-charging power lithium batteries with different technical routes have their own advantages and disadvantages and are suitable for different new energy products.

●Ternary fast-charging batteries are suitable for passenger cars, lithium titanate and other batteries are suitable for buses, and titanium niobium oxide may be a new direction for fast charging.

●At present, fast-charging batteries are widely used in the field of buses. In the future, the consumption structure of fast-charging power lithium batteries will shift to passenger cars and special logistics vehicles.

1. How to understand fast charging?

To understand fast charging, a professional term is the charge and discharge rate C, which can be simply understood as the rate of charging and discharging. The charge and discharge rate of lithium-ion batteries determines how fast we can store a certain amount of energy in the battery, or how fast we can release the energy in the battery.

According to the 2018 new energy vehicle subsidy policy, those with a charging rate less than 3C belong to non-fast charging pure electric buses, and those with a charging rate higher than (including) 3C belong to fast charging pure electric buses. However, the subsidy classification for fast charging is only for new energy buses, and there is no standard for passenger cars and logistics vehicles.

According to the industry and CATL, electric vehicle fast charging refers to a charging method with a charging current greater than 1.6C, that is, a technology that takes less than 30 minutes to charge from 0% to 80%. Based on various opinions, the author proposes that a charging rate less than 1.6C is slow charging, 1.6C-3C is small fast charging, and 3C or above is fast charging. Most electric passenger cars can achieve small fast charging, and the charging rate of fast-charging buses is mostly concentrated in 3C-5C.

If we compare the lithium-ion battery to a rocking chair, the two ends of the rocking chair are the two poles of the battery, and the lithium ions are like excellent athletes, running back and forth at the two ends of the rocking chair. When charging, lithium ions are generated on the positive electrode of the battery, and the generated lithium ions move to the negative electrode through the electrolyte. The carbon as the negative electrode has a layered structure, and it has many micropores for the lithium ions that reach the negative electrode to embed. The more lithium ions embedded, the higher the charging capacity.

During fast charging, lithium ions must be accelerated to embed into the negative electrode instantly. This poses a great challenge to the ability of the negative electrode to quickly receive lithium ions. Batteries of ordinary chemical systems will produce byproducts at the negative electrode during fast charging, affecting the cycle and stability of the battery cell. Energy density and power density can be said to be two directions that are mutually exclusive in the same battery.

Whether it is national policy orientation or company technology layout, high energy density is generally pursued. When the energy density of power lithium batteries is high enough and a car is loaded with enough power, the so-called mileage anxiety can be prevented, and the demand for fast charging will be reduced. However, if the power is large, it will be difficult to be accepted by the market if the cost is not reduced. Therefore, if the battery cost can be controlled, the user anxiety can be greatly alleviated with convenient charging capacity + applicable cruising range, so that fast charging has value.

2. Fast charging application prospects of batteries with different technical routes

The speed of charging is closely related to the overall technical and design requirements of power lithium batteries, charging piles, electric vehicles, power grids, etc., and the biggest influencing factor is still the battery. Let's discuss the application trends of different types of power lithium batteries in the direction of fast charging technology. Almost all kinds of positive electrode materials can be used to make fast-charging batteries, but their applicability and advantages and disadvantages vary.

1. Ternary fast-charging batteries are more suitable for electric passenger cars

Ternary batteries are more valued because of their high energy density. The material itself has excellent conductivity, but the reaction activity is too high, which poses a great challenge to the safety of fast charging.

BAK's latest 3.0 high-energy core launched in May this year has an energy density of nearly 250Wh/kg by introducing silicon-based negative electrode materials, high-nickel positive electrode materials, and specially developed electrolytes, which can achieve an ultra-long range of 500 kilometers. Through the design of charging strategies, the charging time is effectively shortened and the charging efficiency is improved. In extreme emergency mode, charging for 10 minutes can travel 60 kilometers.

According to the usage habits of fuel vehicles, to achieve a full charge within 10-20 minutes, the charging rate must be at least 3-6C. At present, most of the pure electric passenger cars on the market are fully charged to 80% in half an hour to one hour, which has improved a lot compared to the previous two or three hours of charging time, and it is expected to be further compressed to within 20 minutes in the future.

2. Lithium iron phosphate fast charging is available for both passenger and commercial vehicles

Lithium iron phosphate does not have an inherent advantage in the field of fast charging. From the material point of view, the intrinsic conductivity of lithium iron phosphate material is relatively low, only one percent of that of ternary materials. The conductivity of lithium iron phosphate materials must be optimized to meet the needs of fast charging. However, the material cost of lithium iron phosphate is relatively low. Combined with mature technical background and stable product performance, it has a relatively broad application prospect. Representative companies include CATL and Watma.

Limited by the extreme value limit of theoretical energy density, lithium iron phosphate has little room for development in energy density in the future. However, for commercial vehicles such as buses, logistics vehicles, and special vehicles that have already adopted the lithium iron phosphate system, the improvement of energy density is not necessary, and fast charging is increasingly showing its importance.

3. Lithium manganate ion battery is suitable for plug-in hybrid buses

Lithium manganate ion battery has the characteristics of power performance, discharge rate performance, good low temperature performance, and high voltage frequency. In the situation of crazy rise in the upstream raw materials of ternary materials, the cost advantage of lithium manganate is gradually becoming prominent. However, there is still room for improvement in energy density and high temperature performance. In recent years, the proportion of lithium manganese oxide fast-charging batteries in the field of plug-in hybrid buses has increased significantly, with representative companies such as CITIC Guoan Mengguli, Yipeng New Energy, and Microvast Power.

However, the cycle performance of lithium manganese oxide batteries is poor under high temperature conditions. The high temperature performance of lithium manganese oxide batteries can be improved by positive electrode doping, but the modified lithium manganese oxide material is no longer the original lithium manganese oxide. Multi-component composite materials are commonly used in the industry. The positive electrode adopts a mixed system of ternary materials and lithium manganese oxide, and the negative electrode adopts porous composite carbon to further improve the performance of fast charging, but safety still needs to be focused on and continuously improved.

4. Lithium titanate fast-charging batteries are suitable for pure electric buses

Lithium titanate power lithium batteries are named after the negative electrode material, and the positive electrode adopts ternary materials. Zhuhai Yinlong, Microvast Power, and Tianjin Jiewei are typical companies. In terms of performance, lithium titanate ion batteries have excellent low temperature performance, good safety and cycle performance, and the rate performance as a fast-charging battery has also been recognized by the industry. However, there are two prominent problems with lithium titanate at present: First, the energy density is relatively low. Under the pressure of policies and markets that require continuous improvement of energy density, the current market share of lithium titanate in the entire power lithium battery market is relatively low. Second, due to the influence of high-cost small metal materials such as titanium, nickel, and cobalt, the cost of lithium titanate ion batteries is significantly higher than that of other systems.

Lithium titanate ion batteries are significantly better than other systems of fast-charging batteries in terms of cycle life, which is determined by the characteristics of the material itself, that is, the zero strain characteristics. But its disadvantages are obvious, with low energy density, which is only about half of the energy density of the ternary system. In addition, the price is relatively high, and it is currently mostly used in fast-charging buses. In the future, it is urgent to seek higher voltage positive electrode materials and matching electrolytes to solve this defect.

5. New direction of fast charging Titanium niobium oxide negative electrode material

Titanium niobium oxide is developed based on lithium titanate. The main advantage is that the theoretical capacity of titanium niobium oxide is about 280mAh/g, compared with the theoretical capacity of lithium titanate of 175mAh/g.

In addition, the concept of graphene batteries has been quite popular, but there are also controversies in the industry. In lithium-ion batteries, graphene is mainly used as negative electrode active materials and conductive additives. In terms of fast charging capability alone, using graphene as a conductive agent, or using graphene to coat lithium iron phosphate/ternary lithium materials, can achieve better fast charging effects. However, from the perspective of comprehensive cost, process difficulty and other indicators, there are still great challenges.

3. Market prospects for fast charging products

High energy density, fast charging, and low price are the ideal power lithium battery products that users are most looking forward to. However, you can't have your cake and eat it too. Under the existing lithium-ion battery system, the five most important indicators of power lithium batteries, such as rate performance, energy density, life, safety, and price, are fixed in a relatively stable graph. If any one indicator is improved, the other indicators will be relatively lost.

At present, fast-charging power lithium batteries are mainly used in new energy buses, because they have strong selectivity for cities and audience units, that is, cities or units with relatively financial support tend to prefer fast-charging battery buses. However, from the perspective of market development potential, the growth rate and market size of passenger cars and special logistics vehicles will be higher than that of buses in the future, so the consumption structure of fast-charging power lithium batteries will shift to these two types of vehicles in the future.

According to data from Battery my country, my country's production of fast-charging buses in 2017 was 6,486, and the installed capacity of batteries reached 597.52MWh, accounting for 6% of the total new energy buses. Among them, the highest charging rate of fast-charging bus products is 6.42C. The production of models with a rate of 3C-5C was 4,771, and the installed capacity of batteries was 480.68MWh; the production of models with a rate of 5C-10C was 1,715, and the installed capacity of batteries was 116.84MWh. At present, the fast-charging rates of fast-charging buses are mainly concentrated between 3C-5C. From the perspective of battery type, the battery material of fast-charging buses in 2017 was mainly lithium titanate, with an installed capacity of 571.54Mwh, accounting for 95.65%.

According to the shipment volume of four types of power lithium batteries in 2017, 1.54GWh lithium manganese oxide is partially used in plug-in hybrid vehicles and partially meets the requirements of small fast charging, while 16GWh ternary battery vehicles partially meet the requirements of small fast charging. Overall, ternary fast charging batteries are suitable for passenger cars, fast charging batteries such as lithium iron phosphate and lithium titanate are suitable for buses, lithium manganese oxide fast charging batteries are suitable for plug-in hybrid vehicles, and titanium niobium oxide may be a new direction for fast charging. Watma is currently trying to break the current range dilemma by focusing on battery technology innovation and other means. In terms of battery technology, fast charging technology has become Watma's core competitiveness. Watma's high-rate fast charging battery uses small-particle LFP, small-particle artificial graphite, fast charging electrolyte and high-porosity diaphragm to ensure the fast charging performance of the battery. Watma's 32650-5.0Ah battery has achieved a 6C charging constant current ratio of 90%, 6C cycle of 1000 weeks, and 3C cycle of 2000 weeks.

Based on Watma's leading high-rate fast charging technology and shallow charging and discharging mode, pure electric logistics vehicles can use the loading and unloading time to quickly charge for 10 minutes and replenish 30% SOC, which can fully meet the operation needs of electric vehicles. At present, Watma batteries have achieved 6C charging and discharging of single cells and 2-3C charging and discharging of groups, that is, fully charged in 20-30 minutes, and the fast charging speed is more than 2-4 times that of its peers.

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