Requirements for Enabling Extreme Fast Charging of High Energy Density Li-Ion Cells while Avoiding Lithium Plating

Abstract

To improve electric vehicle market acceptance, the charge time of their batteries should be reduced to 10-15 minutes. However, achieving 4C to 6C charge rates with today's batteries is only possible for cells with thin electrodes coming at the expense of low energy density and high battery manufacturing cost. An electrochemical model is validated versus high rate charge data for cells with several loadings. The model elucidates that the main limitations for high energy density cells are poor electrolyte transport resulting in salt depletion within the anode and Li plating at the graphite/separator interface. Next, the model is used to understand what future electrode and electrolyte properties can help enable 4C and 6C charging. Ideally, future electrolytes would be identified with 2X conductivity, 3-4X diffusivity, and transference number of 0.5-0.6. Alternatively charging at elevated temperatures enhances electrolyte transport by 1.5X conductivity and 2-3X diffusivity with a negligible effect on transference number. Another effective strategy to enable 4C and 6C charging is reducing electrode tortuosity. Conversely, increasing electrode porosity and negative/positive ratio are ineffective strategies to improve fast charge capability.

The influence of surface inhomogeneity on the overcharge and lithium plating of graphite electrodes

“…Of note, the ANL authors found that Li plating did not cause measurable discharge capacity loss for currents up to 6C for the conditions studied (see their discussion associated with figure 5 of [7]), despite the fact that their reference electrode measurement confirmed Li plating at the graphite electrode. Recent analyses [7][8][9][10][11][12] of lithium plating underscore its technological relevance. In [13], the authors developed a set of equations that can be used to treat Li plating and subsequent electro-dissolution in the context of a porous electrode, and simulations were provided for a single-particle model, which ignores all impedances associated with the electrolyte phase and considers each particle in the porous electrode to be identically spherical and isolated.…”

Voltage Relaxation to Detect the Onset of Lithium Plating on Graphite for Fast Charging

“…17 Li plating becomes more favorable at higher SOC for a number of reasons, including: the buildup of Li + concentration gradients over time that leads to higher overpotentials near the electrode-separator interface, the decrease of graphite solid state Li diffusion coefficient with increasing Li content, and intraparticle lithiation gradients that result in LiC6 near surface which prevents further intercalation. 25,26 Coulombic efficiency (CE) analysis of the plating onset cycling in Fig. 2c provides strong evidence that dOCV signals correspond to Li plating instead of other graphite phase relaxation processes.…”

Challenges of Fast Charging for Electric Vehicles and the Role of Red Phosphorous as Anode Material: Review

“…The generated heating inside the cell is not completely uniform because the electronic resistance, ionic resistance, and electrochemical reaction static resistance inside the cell are also unevenly distributed, which is accelerated in a fast-charging state. Further study showed that the heat generation normalized of each cell in the pack is approximately 30% greater than that for a single cell, as shown in Figure 4a [14]. The resistance between conductors and plug-ins is also relatively high in fast-charging battery systems.…”

“…To support the fast-charging performance of the cell, considerable effort goes into the design of a single cell battery pack. Generally, the fast-charging performance of a single cell can be improved by adding more conductive agent to the electrode [11], reducing the loading of the electrode material (thinner coating), using a thicker collector, adjusting the suitable tab position, retaining a larger electrode porosity [12,13], ensuring less electrode bending [14], etc. These methods can improve the kinetic and electrical properties and reduce the internal resistance of cells.…”

“…To achieve fast-changing performance with less battery life degradation, systems with different combinations of battery packs and BMSs were studied [14]. As illustrated in Table 2, Case 4, if charged at 350 kW, the value of heat generation per cell (W) was the lowest (80.7 W), while the BMS structure had a pack heat removal of 15 kW and low energy density cells of 175 Wh kg −1 .…”