TianJinlishen, Guoxuan Hi-Tech and other teams have basically achieved the research and development of 300 Wh/kg power batteries. In addition, there are still a large number of units carrying out related development and research work.

The composition of flexible packaging lithium-ion batteries usually includes positive electrodes, negative electrodes, separators, electrolytes, and other necessary auxiliary materials, such as tabs, tapes, and aluminum plastics. According to the needs of the discussion, the author of this paper divides the substances in the soft-pack lithium-ion battery into two categories: the combination of the pole piece unit and the non-energy-contributing material. The pole piece unit refers to a positive electrode plus a negative electrode, and all the positive electrodes and The negative electrode can be regarded as a combination of pole piece units composed of several pole piece units; non-contributing energy substances refer to all other substances except the combination of pole piece units, such as diaphragms, electrolytes, pole lugs, aluminum plastics, protective tapes and terminations. tape etc. For the common LiMO 2 (M = Co, Ni and Ni-Co-Mn, etc.)/carbon system Li-ion batteries, the combination of pole piece units determines the capacity and energy of the battery.

At present, in order to achieve the goal of 300Wh/kg of battery mass specific energy, the main methods include:

(1) Select a high-capacity material system, the positive electrode is made of high nickel ternary, and the negative electrode is made of silicon carbon;

(2) Design high-voltage electrolyte to improve the charge cut-off voltage;

(3) Optimize the formulation of positive and negative electrode slurry and increase the proportion of active material in the electrode;

(4) Use thinner copper foil and aluminum foil to reduce the proportion of current collectors;

(5) Increase the coating amount of the positive and negative electrodes, and increase the proportion of active materials in the electrodes;

(6) Control the amount of electrolyte, reduce the amount of electrolyte and increase the specific energy of lithium-ion batteries;

(7) Optimize the structure of the battery and reduce the proportion of tabs and packaging materials in the battery.

Among the three battery forms of cylindrical, square hard shell and soft-pack laminated sheet, the soft-pack battery has the characteristics of flexible design, light weight, low internal resistance, not easy to explode, and many cycles, and the specific energy performance of the battery is also outstanding. Therefore, the laminated soft-pack power lithium-ion battery is a hot research topic at present. In the model design process of laminated soft-pack power lithium-ion battery, the main variables can be divided into the following six aspects. The first three can be considered to be determined by the level of the electrochemical system and design rules, and the latter three are usually the model design. variables of interest.

(1) Positive and negative electrode materials and formulations;

(2) The compaction density of positive and negative electrodes;

(3) The ratio of negative electrode capacity (N) to positive electrode capacity (P) (N/P);

(4) The number of pole piece units (equal to the number of positive pole pieces);

(5) Positive electrode coating amount (on the basis of N/P determination, first determine the positive electrode coating amount, and then determine the negative electrode coating amount);

(6) The single-sided area of ​​a single positive electrode (determined by the length and width of the positive electrode, when the length and width of the positive electrode are determined, the size of the negative electrode is also determined, and the size of the cell can be determined).

First, according to the literature [1], the influence of the number of pole piece units, the amount of positive electrode coating and the single-side area of ​​a single piece of positive electrode on the specific energy and energy density of the battery is discussed. The specific energy (ES) of the battery can be expressed by equation (1).

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In formula (1): x is the number of positive electrodes contained in the battery; y is the coating amount of the positive electrode, kg/m2; z is the single-sided area of ​​a single positive electrode, m2; x∈N*, y > 0, z > 0; e(y, z) is the energy that a pole piece unit can contribute, Wh, the calculation formula is shown in formula (2).

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In formula (2): DAV is the average discharge voltage, V; PC is the ratio of the mass of the positive electrode active material to the total mass of the positive electrode active material plus conductive agent and binder, %; SCC is the specific capacity of the positive electrode active material, Ah / kg; m(y, z) is the mass of a pole piece unit, kg, and the calculation formula is shown in formula (3).

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In formula (3): KCT is the ratio of the total area of ​​the monolithic positive electrode (the sum of the coating area and the tab foil area) to the single-sided area of ​​the monolithic positive electrode, and is greater than 1; TAl is the thickness of the aluminum current collector, m; ρAl is the density of the aluminum current collector, kg/m3; KA is the ratio of the total area of ​​each negative electrode to the single-sided area of ​​a single positive electrode, and is greater than 1; TCu is the thickness of the copper current collector, m; ρCu is the copper current collector. Density, kg/m3; N/P is the ratio of negative electrode capacity to positive electrode capacity; PA is the ratio of negative electrode active material mass to the total mass of negative electrode active material plus conductive agent and binder, %; SCA is the ratio of negative electrode active material Capacity, Ah/kg. M(x, y, z) is the mass of the non-energy-contributing substance, kg, the calculation formula is shown in formula (4)

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In formula (4): kAP is the ratio of the aluminum-plastic area to the single-sided area of ​​the single positive electrode, and is greater than 1; SDAP is the areal density of the aluminum-plastic, kg/m2; mTab ​​is the total mass of the positive and negative electrodes, which can be seen from is a constant; mTape is the total mass of the tape, which can be regarded as a constant; kS is the ratio of the total area of ​​the separator to the total area of ​​the positive electrode sheet, and is greater than 1; SDS is the areal density of the separator, kg/m2; kE is the mass of the electrolyte and the battery The ratio of the capacity, the coefficient is a positive number. According to this, it can be concluded that the increase of any single factor of x, y and z will increase the specific energy of the battery.

In order to study the significance of the influence of the number of pole piece units, the coating amount of the positive electrode and the single-sided area of ​​the single positive electrode on the specific energy and energy density of the battery, an electrochemical system and design rules (that is, to determine the electrode material and formula, Compaction density and N/P, etc.), and then orthogonally combine each level of the three factors, such as the number of pole piece units, the amount of positive electrode coating, and the single-sided area of ​​a single piece of positive electrode, to compare the electrode material determined by a certain group and the Range analysis was performed on the calculated specific energy and energy density of the battery based on the formula, compacted density and N/P. The orthogonal design and calculation results are shown in Table 1. The orthogonal design results were analyzed using the range method, and the results are shown in Figure 1. The specific energy and energy density of the battery increase monotonically with the number of pole piece units, the amount of positive electrode coating, and the single-sided area of ​​a single-piece positive electrode. Among the three factors of the number of pole piece units, the amount of positive electrode coating, and the single-sided area of ​​a single positive electrode, the amount of positive electrode coating has the most significant impact on the specific energy of the battery; Among the three factors of the single-sided area of ​​, the single-sided area of ​​the monolithic cathode has the most significant impact on the energy density of the battery.

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It can be seen from Figure 1a that the specific energy of the battery increases monotonically with the number of pole piece units, the amount of cathode coating, and the single-sided area of ​​the single-piece cathode, which verifies the correctness of the theoretical analysis in the previous part; the most significant factor affecting the specific energy of the battery is Positive coating amount. It can be seen from Figure 1b that the energy density of the battery increases monotonically with the number of pole piece units, the amount of positive electrode coating, and the single-sided area of ​​a single positive electrode, which also verifies the correctness of the previous theoretical analysis; the most significant factor affecting the battery energy density is the single-sided area of ​​the monolithic positive electrode. According to the above analysis, in order to improve the specific energy of the battery, it is the key to increase the positive electrode coating amount as much as possible. After determining the acceptable upper limit of the positive electrode coating amount, adjust the remaining factor levels to achieve the customer’s requirements; For the energy density of the battery, it is the key to increase the single-sided area of ​​the monolithic positive electrode as much as possible. After determining the acceptable upper limit of the single-sided area of ​​the monolithic positive electrode, adjust the remaining factor levels to meet the customer’s requirements.

According to this, it can be concluded that the specific energy and energy density of the battery monotonically increase with the number of pole piece units, the amount of positive electrode coating, and the single-sided area of ​​a single positive electrode. Among the three factors of the number of pole piece units, the amount of positive electrode coating, and the single-sided area of ​​a single positive electrode, the impact of the amount of positive electrode coating on the specific energy of the battery is the most significant; Among the three factors of the single-sided area of ​​, the single-sided area of ​​the monolithic cathode has the most significant impact on the energy density of the battery.

Then, according to the literature [2], it is discussed how to minimize the quality of the battery when only the capacity of the battery is required, and the battery size and other performance indicators are not required under the determined material system and processing technology level. The calculation of the battery quality with the number of positive plates and the aspect ratio of positive plates as independent variables is shown in formula (5).

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In formula (5), M(x, y) is the total mass of the battery; x is the number of positive plates in the battery; y is the aspect ratio of the positive plates (its value is equal to the width divided by the length, as shown in Figure 2); k1, k2, k3, k4, k5, k6, k7 are coefficients, and their values ​​are determined by 26 parameters related to battery capacity, material system and processing technology level, see Table 2. After the parameters in Table 2 are determined, each coefficient It is then determined that the relationship between the 26 parameters and k1, k2, k3, k4, k5, k6, and k7 is very simple, but the derivation process is very cumbersome. By mathematically deriving the announcement (5), by adjusting the number of positive plates and the aspect ratio of positive plates, the minimum battery quality that can be achieved by the model design can be obtained.

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Figure 2 Schematic diagram of the length and width of the laminated battery

Table 2 Laminated cell design parameters

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In Table 2, the specific value is the actual parameter value of the battery with a capacity of 50.3Ah. The relevant parameters determine that k1, k2, k3, k4, k5, k6, and k7 are 0.041, 0.680, 0.619, 13.953, 8.261, 639.554, 921.609 respectively. , x is 21, y is 1.97006 (the width of the positive electrode is 329 mln, and the length is 167 mm). After optimization, when the number of positive electrode is 51, the battery quality is the smallest.