什么是三元锂电池?

在自然界中，锂元素是最轻的，原子质量最小的金属，它的原子量为6.94g/mol，ρ=0.53g/cm3。锂化学性质活泼，极易失去电子被氧化为Li+，因此标准电极电位最负，为-3.045V，电化学当量最小，为0.26g/Ah，锂元素的这些特点决定了它是一种具有很高比能量的材料。三元锂电池是指采用镍钴锰三种过渡金属氧化物为正极材料的锂二次电池。它充分综合了钴酸锂良好的循环性能、镍酸锂的高比容量和锰酸锂的高安全性及低成本等特点，利用分子水平混合、掺杂、包覆和表面修饰等方法合成镍钴锰等多元素协同的复合嵌锂氧化物。是目前被广泛研究和应用的一种锂离子可充电电池。

三元锂电池寿命

所谓锂电池寿命是指电池在使用过一段时间后，容量衰减为标称容量（室温25℃，标准大气压，且以0.2C放电的电池容量）的70%，即可认为寿命终止。行业内一般以锂电池满充满放的循环次数来计算循环寿命。在使用的过程中，锂电池内部会发生不可逆的电化学反应导致容量下降，比如电解液的分解，活性材料的失活，正负极结构的坍塌导致锂离子嵌入和脱嵌的数量减少等等。实验标明，更高倍率的放电会导致容量更快的衰减，如果放电电流较低，电池电压会接近平衡电压，能释放出更多的能量。

三元锂电池的理论寿命约为800次循环，在商业化的可充电锂电池中属于中等。磷酸铁锂约为2000次，而钛酸锂据说可以达到1万次循环。目前主流的电池厂家在其生产的三元电芯规格书中承诺大于500次（标准条件下充放电），但是电芯在配组做成电池包后，由于一致性问题，主要是电压和内阻不可能完全一样，其循环寿命大约为400次。厂家推荐SOC使用窗口为10%~90%，不建议进行深度充放电，不然会对电池的正负极结构造成不可逆的损伤，若是以浅充浅放来计算的话，循环寿命至少有1000次。另外，锂电池若是经常在高倍率和高温环境下放电，电池寿命会大幅下降到不足200次。

三元锂电池优缺点

三元锂电池在容量与安全性方面比较均衡，是一款综合性能优异的电池。三种金属元素的主要作用和优缺点如下：

Co3+：减少阳离子混合占位，稳定材料的层状结构，降低阻抗值，提高电导率，提高循环和倍率性能。

Ni2+：可提高材料的容量（提高材料的体积能量密度），而由于Li和Ni相似的半径，过多的Ni也会因为与Li发生位错现象导致锂镍混排，锂层中镍离子浓度越大，锂在层状结构中的脱嵌越难，导致电化学性能变差。

Mn4+：不仅可以降低材料成本，而且还可以提高材料的安全性和稳定性。但过高的Mn含量会容易出现尖晶石相而破坏层状结构，使容量降低，循环衰减。

能量密度高是三元锂电池的最大优势，而电压平台是电池能量密度的重要指标，决定着电池的基本效能和成本，电压平台越高，比容量越大，所以同样体积、重量，甚至同样安时的电池，电压平台比较高的三元材料锂电池续航时间更长。单体三元锂电池放电电压平台高达3.7V，磷酸铁锂为3.2V，而钛酸锂仅为2.3V，因此从能量密度角度来说，三元锂电池比磷酸铁锂，锰酸锂或者钛酸锂具有绝对优势。

安全性较差和循环寿命较短是三元锂电池的主要短板，尤其是安全性能，是一直限制其大规模配组，和大规模集成应用的一个主要因素。大量实测表明，容量较大的三元电池很难通过针刺和过充等安全性测试，这也是大容量电池中一般都要多引入锰元素，甚至混合锰酸锂一起使用的原因。500次的循环寿命在锂电池中属于中等偏下，因此三元锂电池目前最主要的应用领域是3C数码等消费类电子产品。

What is a ternary lithium battery?

In nature, lithium is the lightest metal with the lowest atomic mass. Its atomic weight is 6.94g/mol, ρ= 0.53g/cm3。 Lithium has active chemical properties and is easy to lose electrons and be oxidized to Li+. Therefore, the standard electrode potential is the most negative, - 3.045V, and the electrochemical equivalent is the smallest, 0.26g/Ah. These characteristics of lithium determine that it is a material with high specific energy. Ternary lithium battery refers to a lithium secondary battery using three transition metal oxides of nickel, cobalt and manganese as cathode materials. It fully integrates the good cycle performance of lithium cobalate, the high specific capacity of lithium nickalate, the high safety and low cost of lithium manganate and other characteristics, and uses molecular level mixing, doping, coating and surface modification methods to synthesize nickel, cobalt, manganese and other multi-element synergistic composite lithium oxide. It is a kind of lithium ion rechargeable battery that has been widely studied and applied.

Ternary lithium battery life

The so-called lithium battery life means that after a period of use, the capacity of the battery decreases to 70% of its nominal capacity (room temperature 25 ℃, standard atmospheric pressure, and battery capacity discharged at 0.2C), which means that the battery life is terminated. In the industry, the cycle life is generally calculated by the cycle times of full charge and discharge of lithium battery. In the process of use, irreversible electrochemical reactions will occur inside the lithium battery, leading to a decrease in capacity, such as the decomposition of electrolyte, the deactivation of active materials, and the collapse of positive and negative pole structures, leading to a decrease in the number of lithium ions embedded and de embedded. Experiments show that higher rate of discharge will lead to faster capacity attenuation. If the discharge current is low, the battery voltage will approach the balance voltage, which can release more energy.

The theoretical life of ternary lithium battery is about 800 cycles, which is medium among commercial rechargeable lithium batteries. Lithium iron phosphate is about 2000 times, while lithium titanate is said to reach 10000 cycles. At present, the mainstream battery manufacturers promise in their ternary cell specifications that they will charge and discharge more than 500 times (under standard conditions). However, after the cell is assembled into a battery pack, due to the consistency problem, the main reason is that the voltage and internal resistance cannot be exactly the same, and its cycle life is about 400 times. The manufacturer recommends that the SOC use window be 10%~90%, and it is not recommended to conduct deep charging and discharging, otherwise, the positive and negative pole structures of the battery will be irreversibly damaged. If the calculation is based on shallow charging and discharging, the cycle life will be at least 1000 times. In addition, if the lithium battery is often discharged at high rate and high temperature, the battery life will be significantly reduced to less than 200 times.

Advantages and disadvantages of ternary lithium battery

The ternary lithium battery is balanced in capacity and safety, and is a battery with excellent comprehensive performance. The main functions, advantages and disadvantages of the three metal elements are as follows:

Co3+: reduce cation mixing occupation, stabilize the layered structure of materials, reduce impedance value, improve conductivity, and improve cycling and multiplying performance.

Ni2+: It can increase the capacity of the material (increase the volume energy density of the material). Due to the similar radius of Li and Ni, too much Ni will also cause lithium nickel mixed arrangement due to dislocation with Li. The higher the concentration of nickel ions in the lithium layer, the more difficult lithium is to be de embedded in the layered structure, resulting in poor electrochemical performance.

Mn4+: It can not only reduce the cost of materials, but also improve the safety and stability of materials. However, too high Mn content will easily lead to the appearance of spinel phase, which will destroy the layered structure, reduce the capacity and cause cyclic attenuation.

High energy density is the biggest advantage of ternary lithium batteries, and the voltage platform is an important indicator of battery energy density, which determines the basic efficiency and cost of batteries. The higher the voltage platform is, the greater the specific capacity is. Therefore, batteries with the same volume, weight, and even the same ampere hour have longer battery life with higher voltage platform. The discharge voltage platform of single ternary lithium battery is up to 3.7V, lithium iron phosphate is 3.2V, and lithium titanate is only 2.3V. Therefore, from the perspective of energy density, ternary lithium battery has absolute advantages over lithium iron phosphate, lithium manganate or lithium titanate.

Poor safety and short cycle life are the main shortcomings of the ternary lithium battery, especially the safety performance, which has been a major factor limiting its large-scale assembly and large-scale integrated application. A large number of actual measurements show that it is difficult for large capacity ternary batteries to pass the safety tests such as acupuncture and overcharge, which is also the reason why large capacity batteries generally need to introduce more manganese elements, even mixed lithium manganate. The cycle life of 500 cycles is lower than that of lithium batteries. Therefore, the main application field of ternary lithium batteries is consumer electronic products such as 3C digital.