Power battery classification and characteristics according to material system
The current mainstream lithium-ion power batteries are mainly classified according to cathode materials, including lithium iron phosphate, lithium manganese oxide, ternary (mixed nickel cobalt manganese), and lithium iron manganese phosphate; anode materials include graphite and lithium titanate.
Lithium manganate battery (graphite anode) has low specific capacity, but high compaction density, and the overall energy density is equivalent to lithium iron phosphate; the biggest problem is that it is easy to dissolve when working at higher temperatures, and must be doped and improved by surface treatment, but the stability and safety are not as good as lithium iron phosphate. At present, the number of applications in the new energy vehicle market is relatively small.
Lithium manganese iron phosphate (graphite for negative electrode) material has low maturity, high electronic resistance, and currently has a short lifespan. Because there is a mixture of iron-lithium and manganese, there are two stages of platform voltage, and the influence of SOC inconsistency should be considered when making a group strategy.
Lithium iron phosphate battery (the negative electrode is graphite) has a flat voltage platform, good energy utilization, large reserves and good economy. At the same time, there is almost no thermal runaway (thermal runaway temperature is above 800℃), and the material system is very safe. Also because of the safe characteristics of battery materials, lithium iron phosphate batteries can be used as large-capacity single cells (up to several hundred ampere hours), which is conducive to the group efficiency of the system (calculated according to the weight-to-energy ratio, the group efficiency of the hard-shell battery used in passenger cars can reach 78%). Lithium iron phosphate has been widely used in hybrid power, pure electric buses, power grids and home energy storage systems. It is currently the most used lithium-ion power battery in the new energy bus market.
Ternary batteries mostly use a mixture of nickel, cobalt and manganese as the positive electrode material, but also use nickel, cobalt and aluminum as the positive electrode material (such as Tesla’s Panasonic battery). The negative electrode is graphite, which has high energy density (currently NCM batteries can achieve more than 200W·h/kg, and NCA is even higher) and excellent life characteristics. However, if the thermal runaway temperature is above 200°C, it is necessary to consider how to control thermal diffusion in system integration to meet its system safety requirements. Also because of safety considerations, ternary system materials generally do not make large-capacity single cells. Three batteries are mostly used in pure electric passenger vehicles and commercial vehicles that do not carry passengers. It is currently the fastest growing battery for new energy passenger vehicles.
Lithium titanate batteries use lithium titanate instead of graphite as a negative electrode material in a lithium ion battery, and the positive electrode can be the above positive electrode material. Lithium titanate batteries have four major advantages: Good low temperature characteristics (especially charging at -30°C), high power (charging and discharging at a large rate, especially the charging rate can be increased), long cycle life (easy tens of thousands of times), high safety (almost no risk of thermal runaway); Three major shortcomings: low platform voltage (only 2.2V, system is connected in groups, low efficiency), high temperature flatulence problem (now improved, but not completely solved), high cost (about twice the graphite system battery). The battery is mostly used in hybrid power and in scenarios where the single driving range is not high but requires multiple fast charging.
From the perspective of product application, the field of public transportation has the characteristics of large passenger capacity, large power demand, long operating time, high service life requirements, and low energy density sensitivity. In the current stage of large-scale promotion, on the basis of being able to meet the application and cost requirements, in order to ensure maximum safety, the domestic main push for the technical route of lithium iron phosphate. The field of private passenger vehicles has the characteristics of small space, low passenger capacity, unfixed running routes, high driving range, and high sensitivity to energy density. At the same time, compared with public transportation, the passenger car itself has a higher degree of integration of various systems and a better technical platform, which can provide more stringent protection and thermal management for the battery system. At this stage, the ternary technology route is gradually used.
At this stage, lithium-ion power batteries are defined as developmental products in the commercial vehicle and passenger vehicle fields. The ternary material lithium-ion power battery is still in its infancy and can only be produced in small batches, and carry out demonstration operations under the approved areas, scopes and conditions, and conduct real-time monitoring of the operating status of all vehicles in an appropriate manner.