The key performance of lithium batteries includes energy density, charge/discharge rate, cycle life, safety and operating temperature. In this paper, two representative types of lithium batteries for energy storage in new energy power plants are selected and their performance is compared.
By analysing the data in the table, it can be found that the performance differences between LFP and NCM batteries are mainly in the following areas.
Energy density. The energy density of a battery is the amount of electrical energy released per unit volume or mass of the battery, which is largely determined by the performance of the positive and negative electrode materials. Theoretically, the mass energy density of NCM batteries is about 1.5 to 1.8 times that of LFP batteries, but on a product level, the difference in mass energy density between the two types of batteries is not so great in practice due to the different product positioning and safety considerations of each company. For example, the mass energy density of the LFP battery is 155.8 Wh/kg, which is only slightly lower than the 164.7 Wh/kg of the NCM battery, but with the same cell capacity, the NCM battery has a 15% smaller footprint than the LFP battery, which saves land and reduces the cost of transporting and installing the battery.
Charge/discharge multiplier. The charge/discharge rate of mainstream LFP batteries in the market today is 1 C and below, mainly due to the performance and cycle life of the battery. However, for safety reasons, the performance of NCM batteries with a charge/discharge rate of 2 C produced by the same company is usually significantly lower than that of NCM batteries with a charge/discharge rate of 1 C or less, and the mass energy density of some NCM batteries is even lower than that of LFP batteries. Common charge/discharge multipliers for energy storage batteries are 0.25 C (4 h of energy storage), 0.5 C (2 h of energy storage), 1 C (1 h of energy storage). As the cost of lithium batteries has fallen, the 1 C overmatching of storage batteries with lithium batteries is now commonly used to meet the power requirements of energy storage systems with a charge/discharge multiplier of 2 C in early projects. This method is safer and the lithium battery has a longer service life; it is also more economical than a 2 C charge/discharge multiplier system as the 1 C charge/discharge multiplier system can have various profit models such as frequency regulation and peak/valley arbitrage.
Cycle life. At an ambient temperature of 25 °C, a charge/discharge multiplier of 1 C and an EOL of 80%, an LFP battery with a 260 Ah cell capacity has a cycle life of 6000 cycles. This is mainly due to the fact that the LFP battery has a smooth discharge voltage plateau and no phase change during charging and discharging, while the NCM battery has an unstable discharge voltage plateau and phase change during charging and discharging, which can cause the cell capacity to decay and shorten the cycle life. At low charge/discharge rates (below 0.5 C), the cycle life of NCM batteries can be significantly improved, reaching 5000 to 6000 cycles.
At home and abroad, a new portable lithium power station require lithium batteries with a cycle life of 5,500 cycles or 15 years (1 cycle per day), and LFP batteries can meet the cycle life requirement very well.
Safety. Thermal runaway in Li-ion batteries is a chain of heat build-up during the exothermic process of the battery, followed by an increase in battery temperature and an explosion and fire when the intensity and amount of heat generated completely overwhelms the intensity of heat dissipation. Under conditions such as short circuit, local high impedance overheating, extrusion, puncture and impact, the ultimate temperature of NCM batteries can easily reach 200-300°C, generating large amounts of oxygen and making them susceptible to ignition. In the event of thermal runaway, the oxygen in the crystal structure of the LFP battery is in the form of phosphorus-oxygen tetrahedra and no oxygen is released.
However, the cell capacity of energy storage batteries for new portable power stations is much higher than that of passenger car power batteries, and it is more difficult to control the temperature. Therefore, from the safety point of view, energy storage batteries for new energy power stations prefer LFP batteries.
Working temperature. Studies have shown that the discharge capacity of NCM and LFP batteries measured at a high temperature of 55°C has no significant degradation compared to that measured at 25°C, using 25°C as the reference temperature. However, at low temperatures, especially below -20 °C, the discharge capacity of the NCM battery is significantly higher than that of the LFP battery. Therefore, for energy storage systems built in high latitude areas, the low temperature suitability of LFP batteries is not as good as that of NCM batteries due to the large temperature difference between day and night and the low night temperature.
In summary, it can be found that LFP batteries are more suitable for energy storage in new energy power stations.
In this paper, the performance of two mainstream lithium batteries in electrochemical energy storage - LFP batteries and NCM batteries - is compared and analysed, and the results show that LFP batteries are slightly weaker than NCM batteries in terms of mass energy density and low temperature performance, but they have obvious advantages in terms of safety and cycle life. Unlike lithium batteries for passenger car power batteries, energy storage batteries for new portable power stations require higher safety and cycle life, and the operating conditions are relatively mild and do not require high space requirements, so LFP batteries are more suitable.