It’s harder to replace your phone’s lithium ion battery than it is to treat it right in the first place. Many smartphones don’t provide easy user access to their batteries. That includes all iPhones and many flagship Android phones from brands such as Samsung. Official battery replacements can be expensive or inconvenient (try getting an official battery replacement at an Apple Store this year). There are also environmental concerns. Smartphones are, frankly, an environmental disaster and extending the lifespan of your phone battery helps mitigate that.
1. Understand how your phone battery degrades.
With every charge cycle your phone battery degrades slightly. A charge cycle is a full discharge and charge of the battery, from 0% to 100%. Partial charges count as a fraction of a cycle. Charging your phone from 50% to 100%, for example, would be half a charge cycle. Do that twice and it’s a full charge cycle. Some phone owners go through more than a full charge cycle a day, others go through less. It depends on how much you use your phone and what you do with it.

Battery manufacturers say that after about 400 cycles a phone battery’s capacity will degrade by 20%. It will only be able to store 80% of the energy it did originally and will continue to degrade with additional charge cycles. The reality, however, is that phone batteries probably degrade faster than that. One online site claims some phones reach that 20% degradation point after only 100 charge cycles. And just to be clear, the phone battery doesn’t stop degrading after 400 cycles. That 400 cycles / 20% figure is to give you an idea of the rate of decay.
If you can slow down those charge cycles — if you can extend the everyday battery life of your phone — you can extend its battery lifespan also. Basically, the less you drain and charge the battery, the longer the battery will last. The problem is, you bought your phone to use it. You have to balance saving battery life and lifespan with utility, using your phone how and when you want it. Some of my suggestions below may not work for you. On the other hand, there may be things that you can implement fairly easily that don’t cramp your style.

There are two general types of suggestions here. Suggestions to make your phone more energy efficient, slowing battery degradation by slowing down those charge cycles. Reducing screen brightness would be an example of this type of suggestion. There are also suggestions to reduce stress and strain to your battery, affecting its lifespan more directly. Avoiding extremes of heat and cold would be an example of this second type.

2. Avoid extremes of heat and cold. If your phone gets very hot or cold it can strain the battery and shorten its lifespan. Leaving it in your car would probably be the worst culprit, if it’s hot and sunny outside or below freezing in winter.

3. Avoid fast charging.
Charging your phone quickly stresses the battery. Unless you really need it, avoid using fast charging.
In fact, the slower you charge your battery the better, so if you don’t mind slow charging overnight, go for it. Charging your phone from your computer as well as certain smart plugs can limit the voltage going into your phone, slowing its charge rate. Some external battery packs might slow the speed of charging, but I’m not sure about that.

4. Avoid draining your phone battery all the way to 0% or charging it all the way to 100%.
Older types of rechargeable batteries had ‘battery memory’. If you didn’t charge them to full and discharge them to zero battery they ‘remembered’ and reduced their useful range. It was better for their lifespan if you always drained and charged the battery completely.

Newer phone batteries work in a different way. It stresses the battery to drain it completely or charge it completely. Phone batteries are happiest if you keep them above 20% capacity and below 90%. To be extremely precise, they’re happiest around 50% capacity
Short charges are probably fine, by the way, so if you’re the sort of person that finds yourself frequently topping up your phone for quick charges, that’s fine for your battery.
Paying a lot of attention this one may be too much micromanagement. But when I owned my first smartphone I thought battery memory applied so I generally drained it low and charged it to 100%. Now that I know more about how the battery works, I usually plug it in before it gets below 20% and unplug it before completely charged if I think of it.

5. Charge your phone to 50% for long-term storage
The healthiest charge for a lithium ion battery seems to be about 50%. If you are going to store your phone for an extended period, charge it to 50% before turning it off and storing it. This is easier on the battery than charging it to 100% or letting it drain to 0% before storage.

The battery, by the way, continues to degrade and discharge if the phone is turned off and not being used at all. This generation of batteries was designed to be used. If you think of it, turn the phone on every several months and top the battery up to 50%.