- Nexus 5 battery reference is BL-T9
- LG G2 battery reference is BL-T7
- mAh's are different
- Finally the battery connectors looks not at the same places
I think we can conclude that you can't install a G2 battery on a Nexus 5, sorry. However, there is Nexus 5 batteries available on internet. Look for "LG BL-T9" on google and don't miss the Nexus 5 Battery Replacement guide on iFixit.