How does the voltage and current affect the life of the ternary lithium battery during charging?

With the increasing energy density of lithium-ion batteries, traditional lithium cobalt oxide materials are gradually being replaced by higher-capacity ternary materials. Although ternary materials have a layered structure similar to LCO, compared to LCO materials, ternary materials not only greatly improve the material capacity, but also have better thermal stability than LCO materials.

Generally speaking, the ternary materials we often refer to are mainly NMC materials and NCA materials. The capacity of layered materials is affected by the structural stability. Since Ni3+ is more chemically stable than Co elements, The NMC material can also remove more Li during the charging process, so that the material capacity is greatly improved.

In turn, the stability of layered structure of oxide cathode material is influenced by Li number off, too much to take off the Li may result in material layer structure collapse, so in order to ensure the structure stability of the NMC material need to limit of material charge cut-off voltage, ensure that the cycle of long-term stability of the material.

Johannes Kasnatscheew and others of the University of Munster, Germany, on the cycle life and structure of NCM111 and NCM532 (both materials from BMW Group), NCM622 and NCA (two materials from Customcell), NCM811 (from Shanshan Technology) and the effects of stability were studied.

The influence of charge cut-off voltage

NMC material of lithium number is proportional to the charge cut-off voltage, that is to say, the higher the charge cut-off voltage NMC material out of the greater the amount of lithium, the structure of the material and the more unstable accordingly. Below for NCM811 materials under different charge cut-off voltage, the cycle performance curve, you can see the increase after the cut-off voltage, material capacity significantly increased, the acceleration of the material failure followed drop speed.

Comparing the cycle data under different cut-off voltages, it is found that the 4.6V cut-off voltage has the highest specific capacity at the fifth discharge, but after 53 cycles, its capacity drops rapidly, and the NMC111 is lower than 4.5V and 4.4V cut-off voltage capacity. This indicates that the charge cut-off voltage is improved blindly, although the capacity of the material is greatly improved, but the cycle stability of the material is significantly reduced, so it is necessary to select the charge cut-off voltage according to the design life of the battery.

Below for NMC111, NMC532, NMC622, NMC811 and NCA material, under the different cut-off voltage loop 53 times, discharge energy and the discharge energy retention curve, can be seen in the figure, the loop 53 times, discharge energy density is not by the highest voltage of battery, the highest for NMC811 material, in 4.3 V cutoff voltage to obtain the highest discharge energy density, NMC622 and NMC532, NCA materials in charge cut-off voltage of 4.4 V to obtain the highest discharge energy density, NMC111 material won the highest energy density in 4.5 V.

This is only after a 53 cycle of data, with the increase of cycling times, higher cut-off voltage under materials due to the failure of drop speed is faster, according to the above cycle curve trend, as the lowest voltage, discharge energy density will be the highest. In addition from the image below you can see, regardless of whether they are what kind of materials with the increase of charge cut-off voltage can cause capacity decline speed, especially with low Ni content NMC111, NMC532 and NMC622 materials under the influence of cut-off voltage is bigger, this suggests that low Ni content in the material of a few less structure stability.

The influence of ambient temperature

In lithium ion battery actual application, also we need to consider the high temperature stability of the materials, Johannes Kasnatscheew NMC622, NMC811 and NCA materials under normal temperature 60 ℃ and cycle performance is studied, the results shown in the figure below. In general, raise the temperature can improve cell dynamics condition, so as to improve the performance of the battery to this point from the capacity of the battery under 60 ℃ can see obviously, but higher temperatures will have influence on the stability of material circulation.

In 20 ℃ under normal temperature, for example, three kinds of materials in the first 50 times cycle, is close to the cycle of performance, but to increase the temperature to 60 ℃, NMC811 and the capacity of the NCA material circulation after 50 times keep rate is significantly lower than the NMC622 material, indicating that NMC622 material has higher thermal stability.

The influence of forming current on the cycle performance

Intelligent control of the charging system

Johannes Kasnatscheew analyzed the factors affecting the cycling performance of ternary materials, such as the cut-off voltage of charging and the formation voltage and current, as well as the influence of ambient temperature on the cycling performance of NMC and NCA materials. In essence, with the increase of the amount of lithium removal in NMC materials, the structural stability of the materials will decline, affecting the cyclic performance. In addition, the high temperature will also have a negative impact on the stability of the material, thus leading to a decline in the material cycle performance. Johannes Kasnatscheew also designed a new charging system according to the characteristics of NMC material, which cut off the capacity limit and adjust the charging voltage to ensure that the capacity of the battery is the same every time, so as to overcome the decline of charging capacity and discharging capacity caused by the battery over potential and improve the recycling performance of the battery.

The page contains the contents of the machine translation.