Exploring Solid-State Lithium Ion Batteries
Lithium ion batteries are ubiquitous; powering cell phones, power grids, and everything in between. Lithium ion is the most advanced battery technology available today and researchers continue to develop new methods for improving performance and enhancing safety. Solid-state lithium ion batteries, in particular, have garnered growing interest and companies claim they can achieve more than two times the energy density of conventional lithium ion batteries and significantly improve safety. However, solid-state lithium ion batteries are far from achieving commercial viability.
A typical battery cell consists of two solid electrodes separated by a liquid electrolyte. Solid-state batteries replace that liquid electrolyte with a solid one. The interest in using solid electrolytes is to safely utilize lithium metal as the battery anode. Lithium metal has a high theoretical specific capacity of 3860 mah g-1, versus the theoretical specific capacity of the conventionally used graphite anode at 372 mah g-1, as seen in Figure 1. This means that lithium metal can store 10 times more energy than graphite. However, there are major safety concerns when using lithium metal.
Figure 1. Capacity and voltage window comparison of various cathode and anode chemistries.
One such concern is the formation of dendrites, or branch-like growths of lithium metal, which occurs when lithium ions collect in localized areas on the electrode surface. During the charge cycle, ions move from cathode to anode and distribute unevenly on the anode surface. With each subsequent charge cycle, ions find the path of least resistance, causing them to collect in localized areas that protrude from the anode surface. These protrusions can grow long enough to span the distance between the electrodes, causing an internal electrical short circuit and resulting in battery failure. Furthermore, short-circuiting often causes localized heating and, when using a liquid electrolyte with a low thermal stability, that heat can quickly accelerate the onset of thermal runaway.
Replacing the liquid electrolyte with a solid one can physically suppress the dendrite growth. Solid electrolytes can also improve battery safety due to their superior mechanical, electrochemical, and thermal stability when compared to liquid electrolytes. However, the physical limitations of solid electrolytes make them inherently less conductive than their liquid counterparts due to the slowed ion diffusion through the solid medium. The ion conduction of liquid electrolyte is typically on the order of 10-1 S cm-1, while the most conductive solid electrolytes have reported conductivities on the order of 10-3 S cm-1 at room temperature. The ion conductivity of solid electrolytes is thermally dependent and increases with temperature, meaning they are well suited for high temperature applications. However, conductivity also decreases with temperature meaning the energy density of solid-state batteries decreases significantly in cold conditions, demonstrated in Figure 2.
Figure 2. Ion conductivity comparison of various lithium ion battery electrolytes. (Zhao et al., 2015)
In addition to the low ion conductivity, the high interfacial resistance between the electrode and electrolyte surfaces makes ion diffusion even more difficult. The interfacial resistance is due to the poor adhesion and of two solid surfaces and the poor penetration of electrolyte into the porous electrode. With a liquid electrolyte, the electrolyte is free to saturate the electrode structure, utilizing more ions stored deep within the electrode structure. Thus, the electrode-electrolyte interface is greatly reduced when using a solid electrolyte and the number of usable ions to transfer charge is significantly restricted. Typically, the way to overcome this challenge is to introduce a small amount of liquid electrolyte at the electrode-electrolyte interface to reduce that interfacial resistance. This, however, defeats the purpose of using a solid electrolyte to improve battery safety.
Solid electrolytes for solid-state lithium ion batteries are a promising solution for increasing battery capacity while enhancing battery safety, but these general challenges in using a solid electrolyte must be resolved. Solid-state lithium ion batteries are continually improving, but will not be commercially available for years.
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