Cathode electrode composition and micro-structure

The fresh LMO cathode is made by mixing 5% carbon black, 90% LMO active material and 5% PVDF binder together. As shown in the Figure 6.1 (a), (b) and Figure 6.2 (a), carbon blacks are not distributed evenly on the surface of each particle. Instead a considerable portion of carbon blacks aggregate in between LMO particles and form the conductive network. Only a fraction of carbon blacks is in contact with LMO particles. The LMO particle surface is not fully covered by carbon blacks, and part of the surface area is exposed to the electrolyte. The carbon blacks connected to the LMO particles are the bridges for electron transport between the current collector and LMO particles. The LMO surface area exposed to the electrolyte is available for lithium ion intercalation /deintercalation. Figure 6.2 (b) shows the roles of carbon blacks and LMO particle and also the electron pathways in the cathode electrodes. The composition and the micro-structure of the LMO electrode can be simplified and represented by Figure 6.2 (d).

Figure 6.2 FIB-SEM micrograph of (a) a lithium manganese oxide composite electrode.[136] Due to carbon black aggregation, only a fraction of carbon blacks are in contact with the LMO particle (b). Diffusion only model (c) assumes that the carbon blacks are distributed over the particle surface uniformly, and therefore, the electrons and lithium ions react uniformly over the surface, only diffusion law governs the lithium transportation inside the particle. Diffusion & migration model (d) assumes that carbon blacks are not distributed evenly on the surface, and the mass transportation of lithium ions and electrons is controlled by both migration and diffusion.

In the conventional model [27],the lithium transportation in the particles is modeled by only diffusion lay as shown in Figure 6.2 (c). Diffusion only model assumes that the carbon blacks are distributed over the particle surface uniformly, and therefore, the electrons and lithium ions react uniformly over the surface, only diffusion law governs the lithium transportation inside the particle. The diffusion only model has a uniform flux over the particle surface. Therefore, bigger particle surface facilitates the lithium ion and electron reaction, and increases the lithium diffusion. Particle fracture increases the particle surface area, therefore is beneficial for batter performance in the diffusion only model, which is contradictory to the experimental

observations[42]. The electrical isolation cannot be captured in the diffusion only model. In the new proposed diffusion and migration model as shown in Figure 6.2 (d), it’s assumed that carbon blacks are not distributed evenly on the surface, and the mass transportation of lithium ions and electrons is controlled by both migration and diffusion. Therefore, the fracture induced electrical isolation or electrical resistance increase can be captured and the impact of fracture on the battery capacity can be investigated.

As shown in Figure 6.2 (d), when the LiMn2O4/Li half-cell is being charged, the electrons are transferred out from the cathode. The carbon black network provides the pathway for electron transportation. Once the electrons from the LMO particles are out, due to the charge neutrality, the lithium ions are deintercalated out of the particles as well. When the cell is being discharged, the electrons are injected into the current collector from the external circuit and transferred to the LMO particles through the carbon black network. The lithium ions in the electrolyte are also intercalated into particles due to charge conservation.

After cycling, fracture develops as shown in Figure 6.1 (c) and (d). The fractured particles often have parts which are surrounded by fractures and almost isolated from the bulk particles as shown in Figure 6.1 (d). Based on experimental observations, the cathode micro-structure can be simplified and represented by a single-particle half-cell as shown in Figure 6.3.

Figure 6.3 (a) LMO/Li half-cell includes carbon blacks (domain1), LMO particle (domain2), electrolyte and separator (domain3), and lithium metal anode (domain4). (b) Meshed geometry of the single LMO particle with carbon blacks. The carbon blacks are in contact with one side of the LMO surface.

The single-particle model is constructed in COMSOL as shown in Figure 6.3 (a). The model consists of 4 domains, carbon blacks (domain1), LMO particle (domain2), electrolyte and separator (domain3), and lithium metal anode (domain4). In all sections, subscripts 1, 2, 3, 4 denote the domains. Subscripts (1,2), (2,3), (3,4) denote the interfaces between carbon blacks and LMO particle, LMO particle and electrolyte, electrolyte and Li metal anode, respectively.

When the cell is being charged, the current flux is added to the carbon black surface, the electrons are transferred out of the LMO particle through the carbon black network, and lithium ions are deintercalated into the electrolyte. Lithium ions in the electrolyte are transferred to the anode side and deposited on the surface of lithium metal.

Due to the uneven distribution of carbon blacks in the cathode electrode, the carbon blacks are positioned mainly on one side of the LMO particle. Figure 6.3 (b) shows that the carbon blacks are positioned mainly on one side of the particle surface via Matlab random function.

The mass ratio between carbon black and LMO particle is 5%:90%. Due to the van der Waals force, electrostatic force, and Brownian force, LMO particles and carbon blacks dispersed in the liquid phase during electrode fabrication stick to each other, and spontaneously form irregular particle aggregates. Due to carbon black aggregation, carbon blacks are not distributed evenly on the LMO particle surface, instead, they form aggregates in between LMO particles. Therefore, only a fraction of the carbon blacks are in contact with the surface of LMO particles. Here we assume that about 40% of carbon blacks are attached to the surface. Therefore, the mass ratio between LMO particle and carbon blacks is 90:2.