These are by far the most common lithium ion batteries. With some (very rare) exceptions they are all rated 3.7v but actually operate between 3v and 4.2v – any lower or higher and they will be irreparably damaged. They are available in a variety of capacities, up to a maximum (currently) of 3,500mAh.
The 2,600mAh Li-Ion is the sweet-spot of price vs. performance, and I always recommend two sets of 2,600mAh rather than one set of 3,500mAh because it means you can charge one set while using the other, or carry a spare set for those priceless moments when you realise that you forgot to charge the ones in your torch after your last outing!
Lithium ion batteries come in protected and unprotected versions. The former are able to defend themselves against most forms of abuse and carry a one-year warranty, while the latter can't and don't. These batteries also come in regular and high-drain versions, the latter for ultra-high output torches which need currents of more than 6 amps or so.
There's a lot of BS in the battery industry so be careful of Li-Ions that are very cheap or claim capacities of more than 3,500mAh – at best you'll waste your money, at worst they'll burn down your house. This independent website is a great resource on batteries and chargers – especially this "comparator".
Click on the pictures below to see details of my batteries and drop me a line to discuss discounts.