Technically, it's possible to upgrade the battery of LG G8 to a 5000mAh battery, however, it's not recommended because it can damage the device and may void the warranty. The LG G8 comes with a 3500mAh battery which is optimized to work with the device's hardware and software. If you do replace the battery with a higher capacity one, it may cause overheating and may not charge properly, which could ultimately lead to serious damage to the hardware.
In addition, modifying your phone's battery can be complicated, and you may also run into compatibility issues with the other components of your device. It's always best to stick with the original battery that comes with your phone or use a reputable replacement battery from a trusted manufacturer.
If you're experiencing battery issues with your LG G8, there are other steps you can take to improve battery life such as reducing screen brightness, closing unused apps, turning off unnecessary features like Wi-Fi and Bluetooth, and using battery-saving modes. If these steps don't work, you can contact LG customer support or take your phone to an authorized repair center to get the battery replaced.