Why we work with Lithium Iron batteries for home storage

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November 23, 2022

Recently, it has been reported that NIO will upgrade the existing 70kWh NMC ternary battery pack. The new battery pack uses lithium iron phosphate / Lifepo4 batteries, or more than 75kWh, which is planned to be released in the fourth quarter of this year; Tesla CEO Musk said in February, Tesla will replace some of the NMC ternary lithium batteries of electric vehicles with lithium iron phosphate batteries… Currently, the lithium batteries equipped with electric vehicles are mainly lithium iron phosphate batteries and NMC ternary lithium batteries, of which the ternary lithium battery market occupies The rate is high. However, recent signs on the market indicate that the lithium iron phosphate battery market has begun to pick up, impacting the dominant position of NMC ternary lithium batteries.

Lithium iron phosphate batteries are characterized by high safety, high-rate charge and discharge characteristics and long cycle life. The literature shows that under the charging condition of 1C rate charging to 3.65V, then switching to constant voltage until the current drops to 0.02C, and then discharging to the cut-off voltage of 2.0V at 1C rate, the battery capacity still has 80% of the initial capacity after 1600 cycles.

In addition to long life and excellent charge and discharge performance, the biggest advantage of lithium iron phosphate batteries is its safety. The chemical properties of lithium iron phosphate are stable and the high temperature stability is good. No oxygen molecules will be released in the event of impact, needle stick, short circuit, etc., will not produce violent combustion, and have high safety performance.

However, the disadvantage of the lithium iron phosphate battery is that its performance is greatly affected by temperature, especially in a low temperature environment, the discharge capacity and capacity will be greatly reduced. In addition, the energy density of lithium iron phosphate is relatively low. The weight energy density of the battery is only 120Wh/kg. If you calculate the energy density of the entire stack, including the battery management system, heat dissipation and other components, the energy density is even lower.

NMC Ternary lithium battery refers to a lithium battery containing three elements of nickel, cobalt and manganese, transition metal lithium intercalation oxides conforming to the material positive electrode, which can be expressed by the general formula as LiMnxNiyCo1-x-yO2 (0<x<0.5, 0<y<0.5 ). This material combines the advantages of lithium cobaltite, lithium nickelate and lithium manganate to form a three-phase eutectic system of three materials. Due to the ternary synergistic effect, its comprehensive performance is better than any single combination compound. The weight energy density can reach 200Wh/kg.

However, the safety of NMC ternary lithium batteries is poor. The thermal stability of ternary lithium battery is poor, and it will decompose at 250-300℃. It will stop at a point after encountering the combustible electrolyte and carbon materials in the battery. The heat generated will further aggravate the decomposition of the positive electrode. Will deflagrate. In a car accident, an external impact will damage the battery diaphragm and cause a short circuit. The heat emitted during a short circuit will cause the battery to run out of thermal control and quickly increase the temperature to above 300°C, posing a risk of spontaneous combustion. Therefore, for a ternary lithium battery, its battery management system and heat dissipation system are very important.