What Is a Solid State Battery


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A solid state battery is an electrochemical storage device that uses solid electrodes and a solid separator/electrolyte system without any liquid component. In a battery, the anode (negative electrode) and cathode (positive electrode) are separated by the electrolyte. The electrolyte transports ions from one electrode to the other, making possible the chemical reactions inside the battery. The electrolyte can be purely liquid, a gel-type polymer or a solid. Most lithium ion batteries use a liquid electrolyte to carry charge between the anode and cathode. The electrolyte is typically a lithium salt, e.g. lithium hexaflourophosphate (LiPF6), dissolved in an organic solvent which can be dimethyl carbonate (DMC) diethyl carbonate (DEC), ethyl methyl carbonate (EMC), or propylene carbonate (PC). The electrolyte is put in a microporous separator which provides electrical isolation between the cathode and anode while facilitating the transport of lithium ions.

However, the use of liquid electrolytes in lithium ion batteries raises serious safety concerns. Thermal runaways caused by overcharging, overheating, short-circuiting, or physical damages of Li-ion batteries are a huge threat. A thermal runaway event often results in fire or explosions triggered by the ignition of the volatile flammable liquid electrolyte. The flammability characteristics (flashpoint) of common carbonates used to formulate liquid organic solvents vary from 18 to 145 °C. The best performing liquid electrolytes have flashpoints around 30°C. High flammability and low thermal stability of the liquid electrolyte creates challenges for use, storage, and handling.

Solid state batteries are essentially batteries that use solid electrolytes. These batteries are designed with the intention to mitigate or completely eliminate the hazard posed from the use of flammable component and substantially increase the safety of the electrochemical cell. Although the most important incentive for developing solid state batteries originates from their potential to substantially improve safety, the superior thermal stability and mechanical properties enable the use of more energy dense anodes such as lithium metal. The lithium metal is considered to be the ultimate anode for its potential to improve gravimetric energy density and volumetric energy density of a lithium rechargeable battery by up to four and two times, respectively. The charging rate of current lithium-ion batteries is fundamentally constrained by the electrochemistry of the battery system as well as the engineering required to protect the battery from exposure to high temperatures. The close-to-unity lithium-ion transference numbers, negligible concentration polarization, and high thermal conductivity of solid electrolytes allow solid state batteries to be charged at a very fast rate. The life of lithium-ion batteries is also significantly extended with the use of solid state electrolyte system.

In general, there're two types of solid electrolytes: organic solid polymers and inorganic solids. Polymer-based electrolytes are ionically conductive polymers which are typically mixed in composites with ceramic nanoparticles. This allows the electrolyte to possess ideal processability and enhanced electrode/electrolyte interfacial compatibility. However, polymer-based electrolytes require an operating temperature above 60°C to enable practical performance and their mechanical properties are generally insufficient to stabilize the lithium metal anode. Thus, most attention has focused on inorganic solid electrolytes, including oxides and sulfides. Solid electrolytes have fair ionic conductivities at room temperatures, high chemical compatibility vs cathodes and anodes, and a broad electrochemical window.