Do you remember the removable batteries of first generation mobile phone handsets. That was the first onslaught of Lithium-ion batteries on humanity. Li-ion as it was popularly printed on each battery suddenly was a household name. Tech bloggers advised buying a spare battery so that you can quickly swap one while on the move without having the need to hunt for a power outlet. Nowadays, the battery is sealed inside the handset to maintain slim profile. But the batteries inside are still Lithium-ion.
Advantages of Lithium-ion battery
Li-ion batteries are compact. So they can power anything from watches to smartphones. They have a high power density and cost less. When at their peak, they are very efficient. They do not have problem of memory effect. That means it will have no effect while charging a partially charged battery.
Disadvantages of Lithium-ion battery
Li-ion battery suffer from ageing effect. That means over a period of time the battery will lose the ability of getting charged.
Li-ion batteries are dangerous if not designed and handled well. In fact several YouTube videos will show how these batteries catch fire by simply poking in them. Using substandard chargers will also lead to smartphones catching fire. Remember the incident with Samsung Galaxy Note 7, where a tiny bend in the battery led to phones catching fire.
A Li-polymer battery replaces the liquid in Li-ion battery with gel like material. This makes Li-poly battery more flexible and light weight. It is safer because of gel and low chance of leaking. They are typically larger in size and hence most suited for use in laptops.
Lithium-polymer is an old technology. They suffer from memory effect. They do no come in compact forms and hence are not found on smartphones. Though the gel lasts longer, but eventually it gets hardened over a period of time.
But the main problem with Li-poly battery that it is expensive to produce. Combined that with a low power density, you can now see why Li-ion is universally popular despite being unstable. Li-ion batteries also last longer.
Which is better
Though hard to answer, Li-ion batteries with more power density and cheaper manufacturing costs are far ahead in the race. Besides Lithium-polymer is a type of Li-ion battery to begin with, so there is no point in comparing. Which battery to place in a gadget is today decided by manufacturers with nothing left at the end user. But yes, understanding the technology better will help in making the right choice and best use of it.