As a kind of future anode material, silicon has a theoretical capacity of about 4,200 mAh/g, which is more than 10 times higher than the 372 mAh/g of graphite anodes. After its industrialization, it will greatly increase the capacity of the battery. However, silicon is now The main problems with materials are: 1. Volumetric expansion of 300% to 400% during charge and discharge; 2. High irreversible capacity; low Coulomb efficiency leading to actual capacity loss and poor cycle life. After alloying with lithium, silicon crystal volume appears Obvious changes, this volume effect can easily cause the silicon anode material powder, and peel off from the current collector. And due to the silicon volume effect caused by spallation will cause repeated destruction and reconstruction of the SEI, thus increasing the lithium ion Consumption, ultimately affecting the capacity of the battery. At present, the above problems are being solved by means of nanometer silicon powder, silicon carbon coating, doping, and adhesive optimization.
From an engineering point of view, in order to increase the energy density of a battery, the total mass of the electrode or battery needs to be controlled. The quality of the electrode includes an active material, a liquid electrolyte filled in the electrode pores, a binder and a conductive additive, and a current collector. Therefore, the energy density of an electrode depends on the mass ratio of the active material to the non-active material used. Common technical approaches to increase the energy density of a porous electrode include: increasing electrode thickness (active material/current collector ratio) and decreasing porosity (electrolyte/activity) Material ratio. However, due to the limitations of lithium ion transport in the electrode, the increase of electrode thickness and decrease of porosity will decrease the battery power density. In addition, the proportion of mixed with the graphite anode will affect the capacity of the composite electrode and the average volume expansion. Therefore, Optimizing these design parameters is the key to developing high-energy high-power lithium-ion batteries.
Heubner et al. considered the mixing ratio of silicon and graphite, the volume expansion of the material, and determined the optimal design criteria for the porous silicon electrode. They defined the 'deformation threshold', and due to the volume expansion of the silicon negative electrode, the original porosity in the electrode would be filled. To reduce the porosity, in order to avoid violent deformation and stress in contact with the electrode particles during charging, as well as sharp drop in porosity, there is a minimum of the initial porosity of the electrode. When designing the electrode, the porosity must be greater than this value. A 'rate current threshold' is also defined to ensure that the diffusion-limited current is not lower than the required rate current, thereby avoiding a significant reduction in capacity during fast charging. The effect of these design criteria on the performance parameters of the silicon negative electrode is again analyzed. And use the analysis of the standard relationship to optimize the electrode design parameters to ensure the electrode energy density and power density.
1, porosity
The porosity ε0 of the lithium-ion battery electrode can be expressed by Equation (1):
(1)
Vi is the volume of each solid phase component in the electrode, including silicon (Si), graphite (C), binder (B) and conductive agent (A). V is the overall volume of the electrode coating. Assumes SOC = 0 and Between SOC=1, the volume change of each solid component is linear, and the expansion volume of each phase is n times the initial value (the volume expansion of silicon, graphite, conductive agent and binder is nSi=3, respectively). nC = 0.1, nA = 0, nB = 0). Considering this volume expansion, the electrode porosity ε (soc) in different SOC states is given by Equation (2):
(2)
Assuming that the battery's overall expansion is limited to ns times (eg 10%) under the constraints of the battery outer case, the true density of each solid phase ρi (silicon, graphite, conductive agent, binder and electrolyte density respectively For ρSi=2336, ρC=2200, ρA=2200, ρB=1800, ρCC=8920, ρel=1500) and the mass percentage ωi, the equation (3) is obtained:
(3)
According to formula (3), for different initial porosity electrodes, the relationship between electrode porosity and SOC during lithiation is shown in Fig. 1a. Fig. 1b is a schematic diagram of the corresponding microstructure change (assuming that the overall electrode expansion is limited to ns = 10%). As the SOC increases, the porosity decreases significantly. When the initial porosity is in the range of 20-40% (the porosity of a typical commercial graphite electrode), the porosity of the silicon-based electrode will rapidly decrease during charging. Zero. Such a process can cause a large internal mechanical stress in the electrode, causing silicon comminution, electrical contact failure, etc., thereby reducing the capacity. In the case of medium initial porosity (50-70%), the reduction of porosity is not so obvious. However, if you want to maintain SOC = 1, the electrode porosity does not fall to 0, the initial porosity needs to be more than 80%.
Fig. 1 (a) Evolution of porosity during lithiation at different initial porosities; (b) Evolution of electrode porosity at different initial porosities
Fig. 2a shows the relationship between porosity and initial porosity in the lithiation state of the electrode with different silicon contents at different silicon contents. The increase in silicon content leads to a more dense electrode after lithiation, a pure graphite electrode, and a volume expansion of graphite of 10%. The volume change is limited to 10%, and the porosity does not change after lithiation. Figure 2b shows the SOC = 1 lithiated state for three different silicon content electrodes at different electrode volumetric limit values (0%, 10%, 20%). The relationship between the lower porosity and the initial porosity, the smaller the total volume change limit of the electrode, the smaller the porosity of the electrode after lithiation.
Fig. 2 (a) The SOC of the electrode at different silicon contents = 1 The relationship between the porosity and the initial porosity in the lithiated state; (b) The limiting value of the different bulk change (ns) The SOC of the lower electrode = 1 in the lithiated state Relationship between rate and initial porosity
2, electrolyte Li distribution
In the lithiation reaction, lithium ions are inserted into the active material from the electrolyte, and the concentration of lithium in the electrolyte is reduced in the pores of the electrode. A concentration gradient is formed across the entire plate, causing diffusion of lithium to the anode. If the lithium concentration in the electrolyte drops to zero Lithium intercalation reaction stops. Therefore, the maximum current that can be reached, the so-called limiting diffusion current jlim, can be expressed as equation (4), and the effective diffusion coefficient is related to the porosity.
(4)
Fig. 3 Schematic diagram of loss distribution of electrolyte lithium concentration during constant current charging at different porosities
Fig. 3 is a schematic diagram of lithium concentration distribution in the electrolyte at constant current charge at different porosities. (a) Lithium is transported in the electrode at a large porosity and the lithium concentration in the electrolyte is close enough to the initial value. (b) Porosity is decreased. The lithium ion concentration in the electrolyte gradually decreases to form a concentration gradient. (c) The porosity continues to decrease and the lithium concentration inside the electrode approaches 0. (d) Very low porosity, the lithium concentration in the entire electrode rapidly decreases to zero. .
Figure 4 Evolution of the diffusion-limited current rate during lithiation at different initial porosities
Figure 4 shows the evolution of diffusion-limited current rates during lithiation at different initial porosities. With increasing SOC, the rate performance decreases. For example, when the initial porosity ε0=80%, SOC=0. The diffusion-limited maximum current is 9.6C, and SOC=1 is about 0.85C.
Fig. 5 (a) Diffusion limiting magnification and initial porosity under different electrode thicknesses and (b) different silicon contents
Figure 5 shows the relationship between diffusion limiting magnification and initial porosity at different electrode thicknesses and different silicon contents. With the initial porosity increasing, the rate performance improves. At a certain initial porosity, diffusion-limited current increases with electrode thickness. The decrease in. Particularly thick or extremely small electrodes are usually limited by diffusion, the maximum charge-discharge rate drastically decreases when SOC = 1. In addition, increasing the graphite content in the composite can significantly increase the rate capability of the electrode.
Conclusion: Considering the huge volume expansion effect of the silicon negative electrode, the porosity of the electrode will be reduced during the expansion process and the stress between the particles will increase, resulting in powdering. Therefore, for the silicon carbon negative electrode, the battery pole piece design should be better than the graphite negative electrode. larger porosity theoretically calculated, considering the volume and the mass specific capacity, corresponding to the presence of different silicon content than the maximum capacity. in this case the electrode which optimizes the thickness and porosity shown in Table 1. (Analysis: mikowoo)
Table 1 The maximum specific capacity and the corresponding optimal electrode thickness and porosity for silicon-carbon anodes with different silicon contents
