This blog had contributions from Larry Weinstein.
If you are looking to develop application specific batteries, one question you may have is on what type of electrolyte to use or develop. In part 1 of this blog series we described four different types of electrolytes – namely organic, aqueous, room temperature ionic liquids and solid electrolytes – each with its set of advantages and disadvantages. In this blog, we list 6 things for you to consider when looking into the best electrolyte choice for your custom battery.
How to Choose the Right Battery Electrolyte for Your Application
Here are 6 criteria to be considered in identifying or developing the right battery electrolyte for your application:
- Power density – If you need high power density for your application, an electrolyte with high conductivity would better support high current drains. As discussed in part 1 of this blog series, aqueous electrolytes have one or two orders of magnitude higher conductivity than the organic counterparts. Optimizing the cell construction for higher surface area is one way to work around this, if there are other reasons that aqueous electrolytes are not suitable for your application.
- Electrochemical Voltage Window –The electrochemical window of an electrolyte is the voltage range between which the electrolyte is stable. It is obtained by the difference between the cathodic potential and anodic potential. If your application needs to support higher voltages, aqueous electrolytes would not be a very good choice because water would be stable only upto around 2 V. For example let us consider a lithium ion Nickel Manganese cobalt oxide (NMC) battery chemistry, where the potential difference between anode and cathode is 3.7 V. You need an electrolyte that has an electrochemical window of 3.7 V or greater so as to be stable at these voltages. Organic electrolytes, ionic liquids and solid electrolytes all are stable at 4 V, with the latter two going even higher. If you have low voltage electronics you gain nothing by using organic or solid state electrolytes.
- Energy density possible – The energy density of a battery refers to the amount of energy stored per unit weight or volume (W-h/L or W-h/kg). This is a thermodynamic quantity, given by the cell voltage, and the capacity of the limiting electrode material. The electrolyte system chosen determines what electrodes can be used in a battery. Electrode materials have various stability issues in different electrolytes; the specifics of these issues are beyond the scope of this article. As a general rule, organic and solid electrolytes are compatible with a wider array of materials than aqueous electrolytes, and the higher voltage window enables electrode couples with greater energy density.
That said, the conductivity of the electrolyte puts a limit on what sort of battery form factors are possible. For example, bobbin cells in reasonably large form factor (‘C’ and ‘D’ size batteries) are very common for high conductivity aqueous electrolytes, while the lower conductivity of organic electrolytes make large form factor bobbin cells suitable only for very low rate discharge. As noted earlier, for a given power density a lower conductivity electrolyte requires a larger electrode surface area; this higher electrode surface area corresponds to a higher volume fraction of inactive materials (current collector and separator), which reduces the energy density available.
- Temperature range required – Aqueous electrolytes have poor performance at low temperatures, typically, their use is limited to the freezing point of water. Batteries with organic electrolytes operate from -30 to +150 oC, which encompasses the temperature range required by military applications. Solid electrolytes can be used in special applications, as they offer an even wider temperature range of operation (from -40 to +170 oC), which is significantly exceeded only by ionic liquids (from -150 to +260 oC).
- Safety – While aqueous electrolytes, ionic liquids, and solid electrolytes are nonflammable and, hence, safe in operation, organic electrolytes pose safety problems, as sometimes they may burst in fire, or even explode. This is why Li-ion batteries require special transportation and disposal regulations.
- Shelf life – typically, organic electrolytes and solid electrolytes offer much longer shelf life than aqueous electrolytes, in which chemical corrosion and gas evolution processes are difficult to suppress. Solid state electrolytes, by their nature, tend to react only slowly, if at all, with electrodes. Organic electrolytes can be unstable, but they form a passivating layer (known as the solid-electrolyte interphase, or “SEI” for short. Check out this video for a visual representation of SEI formation) on the electrode surfaces, significantly reducing reaction time. By contrast, aqueous systems generally do not form as nice passivating layers as organic electrolytes do, and water decomposition outside of the thermodynamic stability window is harder to suppress than organic electrolyte decomposition.
Clearly, as revealed in Parts 1 and 2 of this blog series on battery electrolytes, there are several factors to consider, when selecting an appropriate electrolyte for your battery application, each of them playing a role in the performance, operating conditions, and safety of your product. As always feel free to contact us for battery development questions or inquiries. You can also check out the battery fundamentals or the battery resources sections of our blog for useful information.