![]() However, silicon swells and contracts dramatically during charge and discharge, causing the silicon particles to pulverize, compromising cycle life. Other anode materials, such as silicon, theoretically offer the possibility to handle higher rates of charge. In the worst case, lithium plating on graphite can lead to the formation of dendrites, root-like structures of lithium that can grow from the anode to the cathode and cause a short-circuit, resulting in failure and potentially presenting a safety hazard This increases cell resistance and accelerates cell degradation. Because the lithium metal reacts with the liquid electrolyte to form chemical reaction side products on the surface of the graphite, it forms a barrier that makes it increasingly difficult for lithium ions to diffuse into the graphite. This plating process reduces the available lithium and lowers the battery’s capacity. When charging speeds exceed the diffusion rate of lithium into graphite, lithium begins plating on top of the graphite particle rather than diffusing into the atomic lattice of the graphite. However, the rate at which lithium can diffuse into the graphite host material is limited by the fundamental properties of the materials themselves. Lithium ions intercalate into the graphite host during charge and are stored there until the battery is discharged. In legacy lithium-ion batteries, most of the material in the anode is graphite. Graphite and other anode materialsĮlectrode loading can restrict fast charging in legacy lithium-ion batteries, and the electrode material also plays a role. In particular, the graphite anode of lithium-ion batteries has proven to be a substantial bottleneck to enabling fast charging speeds in energy-dense cells. To charge a battery with a thicker cathode in the same amount of time as a battery with a thinner cathode requires the former to be capable of handling a higher current density the other components of the battery (the electrolyte, separator and anode) have limited ability to pass high current densities. The current density signifies the magnitude of this flow of lithium ions. The more capacity a cathode offers, the more lithium must cross from the cathode to the anode to fully charge the battery. Not only that, but at very high charge rates this heat can reach unsafe levels if not properly controlled.Įlectrode loading plays another role in affecting charge rates. In addition, a lithium atom moving along a very long and twisted path loses a greater percentage of its energy as heat, which reduces the battery’s efficiency. A thick cathode is like a dense, overgrown forest the lithium cannot easily move through it in a straight line. As the electrode thickness increases, the rate of lithium ions leaving the cathode or entering the anode is restricted, due to increases in the distance the lithium ion must travel and the tortuosity, or twistedness, of the path it takes to move from one electrode to another. This power penalty is the result of several factors. For EV drivers, this means longer charge times and worse driving experience in power-demanding situations, like fast acceleration or driving uphill. However, this limits the transport of lithium ions through the electrode, resulting in inferior power performance. This typically means constructing electrodes with higher mass loading and lower porosity, allowing for more energy-storing active material. This allows for better transport of lithium ions across the battery cell, but at the expense of energy density.įor electric vehicles, on the other hand, the priority has been to maximize range by optimizing the amount of energy a cell can store. For applications like handheld power tools, where high-power output is the priority, cells are typically constructed using electrodes with lower mass of active material (electrode loading) and higher porosity to enable higher electrolyte content. Electrode loadingīatteries are often divided into two general categories: power cells and energy cells. When it comes to charging speed, three features drive these tradeoffs: electrode loading, anode material and temperature. However, the requirements for these distinct applications are very different, and these different requirements necessitate fundamental tradeoffs between power, energy and cycle life. Legacy lithium-ion batteries are versatile they are used today in everything from electric vehicles to power tools. QuantumScape’s technology has been designed to overcome many of these constraints, to unlock a step-change in fast-charging performance that has profound implications for EV adoption and the potential to win over a segment of drivers who might otherwise hesitate to make the switch. ![]() These limitations are largely due to fundamental constraints of battery design. ![]() Fast charging is increasingly important to buyers of electric vehicles, but high-energy legacy lithium-ion batteries are still limited in how fast they can recharge. ![]()
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