iPhone 6s battery issue

[Mme. Defarge]

My phone usually only briefly holds a charge after disconnecting from the charger and often crashes after disconnecting and showing only a 10% charge when reconnected. Then, after rebooting, which is becoming increasingly more difficult, it will be fully charged.

In Settings it shows a maximum battery capacity of 88%. It is also now crashing less with auto-lock disabled.

Any advice other than getting a new phone? I’m hoping to baby this one along for another few weeks, but worry about having a dead phone in case of an emergency.