Energy and power battery design strategies
The design strategy of energy-based batteries is to maximize the proportion of active components as much as possible without sacrificing performance, so as to increase energy density. Generally, the battery structure design is designed with high load, low porosity, large particle size, low conductive agent content, less binder, thin current collector and high binding strength coating, thin separator, high conductivity electrolyte, and thin and narrow single tab.
The design strategy of power batteries is to reduce the internal resistance of the battery as much as possible, so as to improve the rate performance. Generally, the battery structure design is low-load, high-porosity, small-size particles, high conductive agent content, thick current collector with low-resistance coating, thin separator, high-conductivity electrolyte, and thick and wide multiple tabs.
As a result, different batteries will adopt different design strategies. This article compares the design and performance of cylindrical batteries from LG, Samsung, Sony, A123 and other manufacturers in order to understand the battery design methods in the industry.
The table below lists the manufacturers, models, and production dates of the 9 battery models, the rated capacity and maximum continuous discharge current values are taken from the battery specification, and the power-to-energy ratio is shown in W:W hr, but they are calculated based on the maximum continuous discharge current (A) divided by the rated capacity (A hr).
The battery has an initial state of charge (SoC) of 20-40%, and the battery is subjected to a discharge-charge-discharge cycle, and the voltage curve is shown in the figure below. The eight cells have a typical voltage distribution of layered cathode materials such as NMC (LiNi x Mn y Co 1−x−y O2) and NCA (LiNi x Co y Al 1−x−y O2). The only exception is the A123 M1A battery, which is LFP cathode. With the exception of the A123 M1A battery, all batteries slightly exceed their rated capacity, discharge capacity and energy are recorded in the table below.
The discharge energy value of the battery, the cell weight, and the volume are measured to calculate the mass and volume energy density values, as shown in the table below. The table also includes volume and weight power density values, where power is defined as the average discharge voltage multiplied by the maximum continuous discharge current, and then the power density is calculated. The table also includes the total area of the positive electrode obtained by measuring the actual electrode, calculated from the cell capacity divided by the positive electrode area.
The figure below plots the power-to-energy ratio of a battery as a function of areal capacity. There is an inverse relationship between these two parameters, i.e., high-power batteries are designed with low face capacity and low coating weight.
The thickness of the five components is shown in the figure below: negative coating, positive coating, negative copper current collector, diaphragm, and positive aluminum current collector. The Samsung 48G battery has the thickest negative electrode, and the battery provides the highest energy density and the lowest power density. The A123 M1A anode coating is the thinnest, and the battery has the highest power-to-energy ratio, but its positive electrode is thicker due to the low density of LFP. For current collectors, battery designers can take advantage of these thinner materials to increase energy density. For both Sony batteries, the high-power VTC 5A battery has thicker copper and aluminum than the high-energy VTC 6 battery. Thicker current collectors reduce battery resistance and improve heat transfer at the expense of energy density.
As shown in the figure below, except for the A123 M1A battery, which has tabs welded in the middle of the positive and negative electrodes (Figure B), the other 8 batteries are all welded with one tab in the middle of the positive electrode, and two tabs are welded at both ends of the negative electrode.
The actual electrode formulation is unknown and difficult to measure. It is assumed that the ratio of active material:binder:conductive carbon in the positive and negative electrode pieces is 95:4:1 and 96:2:2 (weight), respectively. For the A123 M1A cathode, the electrode formulation is 79:11:10 according to the published patent. The material gram capacity is calculated based on these ratios, the quality of the electrode's coating, and the battery capacity, and the results are shown in the figure below. Among them, the calculated gram capacity of several anodes is higher than the theoretical capacity of graphite of 372 mA hr/g, which indicates that the electrodes contain components with higher capacities, such as silicon.
The average porosity in the electrode can be calculated from the coating weight, coating thickness, and the average density of the coating components, as shown in the figure below. With the exception of the Sony VTC 6 battery, the negative porosity values are very similar. The positive electrode porosity varies greater, and the three cells with the highest power-to-energy ratio (M1A, HB2, and HB4) have higher porosity than the others.
The amount of electrolyte per Ah of the battery is shown in the figure below, the electrolyte should fill all the holes in the electrode and separator, but there is still some void space left in the battery that is not filled by the electrolyte, and it is generally considered that the amount of electrolyte of 3g/Ah is reasonable. Batteries that use thinner separators and lower porosity cathodes can reduce this value to <2g/Ah. The thick and relatively porous LFP cathode in the A123 battery requires more electrolyte.
The morphology of the positive and negative electrodes of the battery is shown in the figure below, except for the LFP morphology of A123, the other batteries are similar.
negative electrode
cathode
The electrode EDS spectrum was also tested, and the active substances of the battery were confirmed in various aspects, as shown in the table below.
The size of the positive and negative secondary particles was estimated from SEM image measurements. The graph below plots the typical particle size vs. the power-to-energy ratio, and the results show that higher power batteries use smaller particle size materials, as expected.
In conclusion, the design of the battery and electrodes affects the power and energy density of the lithium-ion battery. High power density batteries require lower electrode coating weight, thinner separators with lower bending, and thicker tabs and current collectors to minimize battery resistance. Electrode resistance and lithium ion diffusion path length can be reduced by using smaller active material particle sizes and more conductive additives. Each type of battery needs to be designed according to the application scenario and specific needs.
Bibliography:
Lain,Brandon,Kendrick.Design Strategies for High Power vs. High Energy Lithium Ion Cells[J]. Batteries, 2019, 5(4):64.DOI:10.3390/batteries5040064
Source: Lithium wants to live
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