NiCd (Nickel-cadmium Battery), NiMH (Nickel-metal-hydride Battery)

Both NiCd and NiMH use nickel hydroxide for the positive electrode. NiCd uses cadmium for the negative electrode, NiMH uses a hydrogen storage alloy, and the electrolyte is an aqueous solution mainly composed of KOH (potassium hydroxide). Although both NiCd and NiMH can be used for high-power applications such as motors, NiCd is being replaced with the safer NiMH because NiCd contains cadmium that causes pollution-related disease. Now, most of the rechargeable batteries are NiMH. Batteries like dry-cell, for example, Eneloop and evolta, are also NiMH.

As a feature of NiMH, it is resistant to repeated charging and discharging and has high safety. On the other hand, NiMH has a strong memory effect, and there is a problem that the amount of electrical power that can be generated decreases as the battery charge is continued. To reset this memory effect, it must be fully discharged and then recharged.

LiB (Lithium Ion Storage Battery)

Lithium ion storage battery is used in various products such as laptop computers, smartphones, and tablets. You may think that almost all LiB is used in batteries that exist around you. It is also widely expected as a battery for electric vehicles. Since LiB has a very high energy density, about 2.5 times that of NiMH, it is an essential storage battery for reducing the weight of mobile devices such as mobile phones. In addition, since there is no memory effect, it is also an advantage that it is possible to add charge, and it does not self-discharge very much even if it is left for a long time.

Table. Self-discharge Rate of Each Battery

battery type	self-discharge rate [%/month]
lead storage battery	3 to 20
nickel-cadmium battery	20 to 45
nickel-metal-hydride battery	15 to 40
lithium-ion battery	1 to 5

However, because the energy density is high, even a slight voltage fluctuation results in overcharging, which leads to extremely shortening the battery life. In addition, if overcharging or over discharging is repeated, the internal pressure of the battery will rise, causing a burst or fire accident. You may have seen the news that smartphone has exploded and caused a major burn, or that the aircraft has exploded a battery.

Solid-state Battery

One of the problems with the above batteries is "leakage". If it is left for a long time, the electrolyte contained in battery may leak out. Leakage can cause a short circuit, and in some cases, can damage the internal circuitry. Leakage occurs because an aqueous solution is used for the electrolyte, and to avoid this, a solid-state battery with a solid electrolyte has been developed. The advantage is that it is easy to handle, and it is less likely to cause an accident.

On the other hand, there is also a drawback. The solid-state battery developed by Toyota for electric vehicles is a lithium ion storage battery that uses a sulfide-based solid electrolyte and a layered oxide positive electrode, and currently has a volumetric energy density of only about 200 Wh/L. This is less than half. In order to extend the cruising distance, it is necessary to raise the volumetric energy density to 400 to 600 Wh/L, and its development is currently in progress.

Metal-air Battery

A metal-air battery uses oxygen in the air for the positive electrode and metal for the negative electrode. It is used for small and light electric appliances such as hearing aids and kitchen timers. Alkali metals such as Li, Na, and Ca and alkaline earth metals are chemically reacted with oxygen in the air to extract electricity.

Therefore, when not in use, keep it shielded to block oxygen and peel off the shield when it is used. Battery performance varies depending on the metal (element) used, and the merit is that it can be downsized because it does not require an electrolyte. On the other hand, it is difficult to develop as a rechargeable battery because metal causes dendrite growth at the time of charging, and an infrastructure for recycling is necessary.

Fuel Battery

Fuel batteries are fundamentally different from the rechargeable batteries introduced so far. Basically, you can think of it as the same as a generator. In other words, by continuing to provide some kind of fuel, the generator is continued generating electrical power.

Though there is an impression that is not likely to be around ours, Panasonic's "ENE-FARM" is a typical example of a household fuel battery. The hydrogen that is provided as fuel and the oxygen in the air react with each other to produce water and generate electrical power. Electrical power is generated by the reverse reaction of the water electrolysis experiment conducted in science experiments.

However, since hydrogen has a high reaction rate and needs attention in handling, for example, for a fuel battery considered for mounting on an electric vehicle, it should not be accumulated even if hydrogen leaks or hydrogen does not leak to the surroundings.