For electric vehicle users, the biggest concerns are charging time and driving range. With current technology, it's difficult to achieve both simultaneously. Therefore, two main approaches have emerged in the development of power batteries: one focuses on increasing the specific energy of lithium-ion batteries to extend the driving range; the other focuses on reducing charging time through fast charging, improving the fast-charging performance of lithium-ion batteries. With technological advancements and in-depth research into lithium battery materials, the challenges previously encountered by fast-charging technology may soon be resolved.
How to understand fast charging?
To understand fast charging, one technical term is unavoidable—charge/discharge rate (C), which can be simply understood as the rate of charging and discharging. The charge/discharge rate of a lithium-ion battery determines how quickly we can store energy in the battery or how quickly we can release energy from the battery.
According to the 2018 new energy vehicle subsidy policy, pure electric buses with a charging rate of less than 3C are classified as non-fast charging pure electric buses, while those with a charging rate of 3C or higher are classified as fast charging pure electric buses. However, the subsidy classification for fast charging only applies to new energy buses, and there are no standards for passenger cars and logistics vehicles.
According to industry standards and the definition of CATL (300750), fast charging for electric vehicles refers to a charging current greater than 1.6C, meaning a charging time from 0% to 80% is less than 30 minutes. Based on various opinions, this author proposes that charging rates less than 1.6C are considered slow charging, 1.6C-3C are considered small fast charging, and above 3C are considered fast charging. Most electric passenger vehicles can achieve "small fast charging," while fast-charging buses mostly have charging rates concentrated between 3C and 5C.
If we figuratively compare a lithium-ion battery to a rocking chair, with the two ends of the chair representing the battery's electrodes, then lithium ions are like excellent athletes running back and forth between the two ends. During charging, lithium ions are generated at the positive electrode and then move through the electrolyte to the negative electrode. The carbon at the negative electrode has a layered structure with many micropores, allowing the arriving lithium ions to intercalate. The more lithium ions intercalated, the higher the charging capacity.
During fast charging, lithium ions need to be rapidly and instantaneously inserted into the negative electrode. This poses a significant challenge to the negative electrode's ability to quickly accept lithium ions. In batteries with conventional chemical systems, byproducts appear at the negative electrode during fast charging, affecting the cell's cycle life and stability. Energy density and power density are, in a sense, two mutually exclusive goals within the same battery.
Both national policy guidance and corporate technology strategies generally pursue high energy density. When the energy density of power batteries is high enough, a vehicle can carry a large enough battery, avoiding so-called "range anxiety" and reducing the demand for fast charging. However, if the cost doesn't decrease, a large battery capacity will be difficult for the market to accept. Therefore, if convenient charging capabilities combined with suitable driving range can be achieved while controlling battery costs, user anxiety can be greatly alleviated, thus giving fast charging its value.
Fast charging application prospects of batteries with different technologies
The charging speed is closely related to the overall technology and design requirements of the power battery, charging pile, electric vehicle, and power grid, with the battery being the biggest influencing factor. We will specifically discuss the application trends of different types of power batteries in fast-charging technology. Almost all cathode materials can be used to manufacture fast-charging batteries, but their applicability and advantages/disadvantages vary.
1. Ternary fast-charging batteries are more suitable for electric passenger vehicles.
Ternary lithium batteries are highly valued for their high energy density and excellent conductivity, but their high reactivity poses a significant challenge to the safety of fast charging.
Representative companies in the ternary lithium battery fast charging system include CATL and BAK. CATL has developed "superconducting electronic grid" and "fast ion ring" technologies, which can charge the SOC from 5% to 85% within 15 minutes, with an energy density of 190Wh/kg and a cycle life of over 2500 cycles. Its main application area is passenger vehicles, and it is expected to have the capability for mass production in 2018.
BAK's latest 3.0 high-energy chip, launched in May this year, utilizes silicon-based anode materials, high-nickel cathode materials, and a specially developed electrolyte, achieving an energy density of nearly 250Wh/kg and a range of up to 500 kilometers. Through its charging strategy design, it effectively shortens charging time and improves charging efficiency. In extreme emergency modes, a 10-minute charge provides 60 kilometers of range.
Based on the usage habits of gasoline vehicles, to achieve a full charge within 10-20 minutes, the charging rate needs to be at least between 3-6C. Currently, most pure electric passenger vehicles on the market can be charged to 80% in half an hour to an hour, which is a significant improvement over the previous two to three hours of charging time. In the future, it is expected to be further reduced to within 20 minutes.
2. Lithium iron phosphate fast charging is compatible with both passenger and commercial vehicles.
Lithium iron phosphate (LFP) does not have an inherent advantage in the fast charging field. From a material perspective, the intrinsic conductivity of LFP is relatively low, only one percent of that of ternary materials, requiring conductivity optimization to meet the demands of fast charging. However, LFP has relatively low material costs, and combined with mature technology and stable product performance, it has broad application prospects. Representative companies include CATL and Wotema.
Limited by the theoretical maximum energy density, lithium iron phosphate (LFP) has limited room for improvement in energy density in the future. However, for commercial vehicles such as buses, logistics vehicles, and special-purpose vehicles that already use LFP systems, increasing energy density is not essential, while fast charging is becoming increasingly important.
3. Lithium manganese oxide batteries are suitable for plug-in hybrid buses.
Lithium manganese oxide batteries possess advantages in power performance, discharge rate performance, low-temperature performance, and high voltage frequency. Furthermore, with the soaring prices of upstream raw materials for ternary lithium batteries, the cost advantage of lithium manganese oxide is gradually becoming more apparent. However, improvements are still needed in energy density and high-temperature performance. In recent years, the market share of lithium manganese oxide fast-charging batteries in plug-in hybrid buses has grown significantly, with representative companies including CITIC Guoan (000839) Mengguli, Yipeng New Energy, and Microvast Power.
However, lithium manganese oxide batteries exhibit poor cycle performance at high temperatures. While doping the positive electrode can improve their high-temperature performance, the modified lithium manganese oxide material is no longer the "original lithium manganese oxide." The industry commonly uses "multi-component composite materials," employing a mixture of ternary materials and lithium manganese oxide for the positive electrode and porous composite carbon for the negative electrode. This further enhances fast-charging performance, but safety remains a key concern and requires continuous improvement.
4. Lithium titanate fast-charging batteries are suitable for pure electric buses.
Lithium titanate (LTT) batteries are named after their negative electrode material, while the positive electrode uses ternary materials. Typical companies involved include Zhuhai Yinlong, Microvast, and Tianjin Jiewei. In terms of performance, LTT batteries exhibit superior low-temperature performance, good safety, and good cycle life. Their rate performance as fast-charging batteries is also recognized by the industry. However, LTT currently faces two prominent issues: First, its energy density is relatively low. Under pressure from both policy and market demands for continuously increasing energy density, LTT's current market share in the overall power battery market is relatively low. Second, due to the high cost of small metal materials such as titanium, nickel, and cobalt, LTT batteries are significantly more expensive than other battery systems.
Lithium titanate batteries exhibit significantly better cycle life than other fast-charging battery systems, thanks to the inherent "zero-strain" characteristic of the material. However, they also have significant disadvantages, including lower energy density, which is only about half that of ternary systems. Furthermore, their relatively high price limits their current application primarily to fast-charging buses. There is an urgent need to find higher-voltage cathode materials and matching electrolytes to address these shortcomings.
5. A New Direction in Fast Charging – Titanium Niobium Oxide Anode Materials
Titanium niobium oxide was developed based on lithium titanate. Its main advantage is that, compared to the theoretical capacity of lithium titanate (175 mAh/g), the theoretical capacity of titanium niobium oxide is around 280 mAh/g.
In October 2017, Toshiba officially announced the successful development of a new generation of automotive lithium-ion batteries, expected to be commercially available in 2019. This battery uses titanium niobium oxide material, representing a revolutionary advancement compared to current ternary and lithium iron phosphate technologies. The new battery boasts advantages such as high energy density and fast charging efficiency, reaching 90% charge in just 6 minutes and providing a range of 320 kilometers. Currently, lithium batteries require an average of 30 minutes to charge to 80%.
Furthermore, the concept of "graphene batteries" has been quite popular, but it is also controversial within the industry. In lithium batteries, graphene is mainly used as a negative electrode active material and a conductive additive. In terms of fast charging capability alone, using graphene as a conductive agent, or coating lithium iron phosphate/ternary lithium materials with graphene, can achieve good fast charging performance. However, considering factors such as cost and manufacturing complexity, it still presents significant challenges.
Market Prospects for Fast Charging Products
High energy density, fast charging, and low price are the ideal power battery products that users most expect. However, "you can't have your cake and eat it too." Under the current lithium-ion battery system, the five most important indicators of power batteries—rate performance, energy density, lifespan, safety, and price—are fixed in a relatively stable curve. Improving any one indicator will result in a relative loss in the others.
Currently, fast-charging batteries are mainly used in new energy buses because they are highly selective in terms of cities and target units; that is, cities or units with relatively sufficient financial resources tend to prefer buses with fast-charging batteries. However, in terms 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. Therefore, the future consumption structure of fast-charging batteries will shift towards these two types of vehicles.
According to data from Battery China, in 2017, my country produced 6,486 fast-charging buses, with a battery capacity of 597.52 MWh, accounting for 6% of the total new energy buses. Among these, the highest charging rate for fast-charging buses was 6.42C. Buses with charging rates between 3C and 5C produced 4,771 units, with a battery capacity of 480.68 MWh; those with charging rates between 5C and 10C produced 1,715 units, with a battery capacity of 116.84 MWh. Currently, the fast-charging rates for buses are mainly concentrated between 3C and 5C. In terms of battery type, lithium titanate was the primary battery material used in fast-charging buses in 2017, with an installed capacity of 571.54 MWh, accounting for 95.65%.
Based on the 2017 shipment volumes of four types of power batteries, 1.54 GWh of lithium manganese oxide batteries were used in plug-in hybrid electric vehicles, partially meeting the requirements for small-scale fast charging, while 16 GWh of ternary lithium batteries partially met the requirements for small-scale fast charging in electric vehicles. Overall, ternary lithium fast-charging batteries are suitable for passenger cars, lithium iron phosphate and lithium titanate fast-charging batteries are suitable for buses, lithium manganese oxide fast-charging batteries are suitable for plug-in hybrid electric vehicles, and titanium niobium oxide may be a new direction for fast charging.
