High energy density, long cycle life, durability and safety are among the chief concerns of battery manufacturers today. While conventional lithium-ion liquid electrolyte batteries have enjoyed market domination across industries ranging from portable electronics to electric cars, issues with safety, expensive sealing agents, and catastrophic failure modes caused by the liquid electrolyte have shown that the technology has plenty of room for improvement. Replacing the liquid electrolyte with a solid could be the solution the battery industry has been looking for - here's everything you need to know about solid state batteries.
Solid state batteries are batteries that forgo the conventional liquid electrolyte for a solid one that is to say a battery made up entirely of solid components. As with conventional batteries, they are made up of a cathode, an anode and an electrolyte. The primary difference lies in the mechanism through which ions travel from one electrode to another through a solid electrolyte membrane.
Regardless of chemistry, solid state batteries use redox reactions to store and deliver energy. Oxidation occurs at the anode, reduction occurs at the cathode and the battery is able to use this phenomenon to store energy (charge) and release it (discharge) as necessary. During discharge, ions travel through an ion-conductive solid matrix instead of the ionic salt saturated solvent state of typical liquid electrolytes.
Solid state electrolytes are fast ion conductors solids that allow ions to move freely throughout the solids crystalline matrix. Fast ion conductors are best thought of as a material that lies between crystalline solids that possess a regular structure with fixed ions and structure-less liquid electrolytes with freely flowing ions. Solid electrolytes often come in the form of gels, glasses and crystals with novel internal structures. In solid state batteries, solid electrolytes must meet a combination of high ionic conductivity, low internal resistance and high electronic resistance. The higher the ionic conductivity is the better the power density and the lower the internal resistance of the battery. The better insulating the solid electrolyte is to electrons, the lower the self-discharge rate and the higher the charge retention. Choice of solid electrolyte depends on the chemistry of the battery, and the ions available for conduction. In the case of lithium ion solid state batteries, a solid electrolyte like LiI/Al2O3 is an excellent Li+ conductor.
The best way to understand why solid state batteries are so exciting is to look at the problems caused by liquid electrolyte in lithium ion batteries on the market today. Much of the bulk found in lithium ion batteries is due to separator systems and safety precautions required to deal with the catastrophic failure modes of lithium batteries. Let's take a look at some of the more pressing problems scientists hope this technology will be able to solve.
The most obvious advantage of solid state batteries is the avoidance of electrolyte leakage. If you've ever had to deal with the messy aftermath of some old AA batteries left behind in an old toy, you're already somewhat familiar with the problem. In order to function, the battery needs a medium through which ions can be transferred during discharge and charge. If a cell dries out, due to exposure, rupture the battery will no longer be able to function. In higher rate applications electrolyte leakage can be devastating, creating a fire hazard, providing paths for electrical shorts and other problems. Using a solid electrolyte inherently avoids this failure mode. Solid state batteries can help manufacturers by removing the need for advanced sealants, pressurizing electrolyte and including flame retardant failsafes.
In batteries, a thermal runaway reaction is a series of cascading exothermic reactions that are accelerated by an increase in temperature that occurs when a cell rapidly discharges its stored energy. The consequences of this reaction are rising internal cell temperature, rising pressure, venting of flammable gases in the liquid electrolyte and the risk of explosion and shrapnel. The liquid electrolyte in lithium ion cells is highly flammable, and leakage due to rupture can lead to disastrous consequences especially in scenarios like an automobile crash. Replacing the flammable liquid with a solid electrolyte can prevent thermal runaway from occurring.
Cycle life or the total number of charge/discharge cycles a battery can perform is the main metric used by the industry to judge the operating life of a battery. A key limiter on conventional liquid electrolyte batteries is the tendency for metal deposits to form within the battery during charge. These deposits can form dendrites which penetrate through separator material and potentially cause a short. On a fundamental level, liquid electrolytes are also attacking the electrodes within the battery themselves. The metals will slowly dissociate into the surrounding liquid medium over time, with the ebb and flow of electrolyte during cycling. The more cycles a cell experiences, the more deposits will inevitably form within the cell leading to a short. A solid electrolyte avoids this problem entirely allowing the cells to survive hundreds of thousands of cycles.
While solid state batteries have been around for a long time, it is only in recent years that the technology has started to make some sizeable steps towards commercial applications. Advances in material science, computer modeling techniques, electrochemistry and manufacturing have opened up new possibilities to the battery industry. More recently, the British electronics giant Dyson invested $15 million USD into the Michigan based solid state battery company Sakti3, following the ranks of General Motors, Khosla Ventures, Itochu and Khosla Ventures among others for a total of $50 million as of March, 2015. Sakti3 has remained tight-lipped on the exact materials used in their solid state lithium ion battery technology, but their use of large advanced computer modeling algorithms coupled with their focus on manufacturability and process technology has earned them intense interest from major investors in the industry. The future is bright for solid state batteries.