The core of new energy pure electric vehicles is undoubtedly the battery. Nowadays, the batteries of mainstream new energy pure electric vehicles on the market basically use ternary lithium batteries and lithium iron phosphate batteries. Relevant data shows that in 2017 the Ministry of Industry and Information Technology announced Among the 8 batches of 296 new energy passenger vehicles, 221 models use ternary lithium batteries, while only 33 use lithium iron phosphate.
Lithium iron phosphate battery: refers to a lithium-ion battery using lithium iron phosphate as the cathode material. Its characteristics are that it does not contain precious elements such as cobalt, the price of raw materials is low, and the resources of phosphorus and iron are abundant in the earth, so there will be no supply problems. It has a moderate operating voltage (3.2V), large capacity per unit weight (170mAh/g), high discharge power, fast charging, long cycle life, and high stability in high temperature and high thermal environments.
Lithium iron phosphate batteries are characterized by high safety, high rate charge and discharge characteristics and long cycle life. Literature shows that the charging conditions are charging to 3.65V at a 1C rate, then switching to constant voltage until the current drops to 0.02C, and then discharging at a 1C rate to a cut-off voltage of 2.0V. After 1,600 cycles, the battery capacity still remains 80% of the initial capacity. Lithium iron phosphate batteries have at least the following five advantages: higher safety, longer service life, does not contain any heavy metals and rare metals (low raw material cost), supports fast charging, and has a wide operating temperature range.
The disadvantage of lithium iron phosphate batteries is that their performance is greatly affected by temperature. Especially in low-temperature environments, the discharge capacity and capacity will be greatly reduced. In addition, the energy density of lithium iron phosphate is low. Calculating only the weight of the battery, the energy density is only 120Wh/kg. If the energy density of the entire stack, including the battery management system, heat dissipation and other components, is calculated, the energy density of DuPont titanium dioxide is even lower. It is far from meeting the requirements clearly stated in the “Energy Saving and New Energy Automobile Industry Development Plan (2012-2020)” issued by the State Council that “the energy density of battery modules is greater than 150 Wh/kg”. In addition, the preparation cost of lithium iron phosphate battery materials and the manufacturing cost of batteries are high, the battery yield is low, and the product consistency is poor.
Ternary lithium battery: a lithium battery using lithium nickel cobalt manganate (Li(NiCoMn)O2) ternary cathode material as the positive electrode material. This material combines the advantages of lithium cobalt oxide, lithium nickel oxide and lithium manganate to form a three-phase eutectic system of the 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. According to Ouyang Minggao of Tsinghua University, the “ternary” material referred to in this survey refers to the common term “ternary power battery” in which the positive electrode is ternary and the negative electrode is graphite. And in In actual research and development applications, there is another type of material with a ternary positive electrode and lithium titanate negative electrode, which is usually called “lithium titanate”. Its performance is relatively safe and its life is relatively long. It does not belong to the commonly known “ternary materials”. “
The advantages of ternary lithium batteries are high energy density and better cycle performance than normal lithium cobalt oxide. At present, with the continuous improvement of the formula and the improvement of the structure, the nominal voltage of the battery has reached 3.7V, and the capacity has reached or exceeded the level of lithium cobalt oxide batteries.
The safety of ternary lithium batteries is poor. Due to the unstable high-temperature structure of nickel-cobalt-aluminum, the thermal stability is poor, and the pH value is too high, which can easily cause the monomer to bloat, thus causing danger. Decomposition will occur at 250-300°C. When encountering the flammable electrolyte and carbon materials in the battery, it will ignite immediately. The heat generated will further intensify the decomposition of the positive electrode, and it will deflagrate in a very short time. In a car accident, external impact will damage the battery diaphragm, leading to a short circuit. The heat emitted during the short circuit will cause thermal runaway of the battery and rapidly raise the temperature to over 300°C, posing a risk of spontaneous combustion. Therefore, for ternary lithium batteries, their battery management system and heat dissipation system are crucial.
So far, there has been no real winner between ternary and lithium iron phosphate batteries. Of course, ternary lithium batteries have the slight upper hand, but neither is a perfect solution at least at this stage. Graphene Or other alternative energy technologies such as hydrogen fuel cells are eyeing it.
