BATTERY SEPARATOR, LITHIUM BATTERY AND METHODS FOR PRODUCING THEM

Provided are a battery separator with less voids, a lithium battery comprising the battery separator, and methods for producing them. A battery separator comprising an oxide electrolyte sintered body and a resin, wherein the oxide electrolyte sintered body has grain boundaries between crystal particles of a garnet-type ion-conducting oxide; wherein a number average particle diameter of the crystal particles is 3 μm or less; and wherein the oxide electrolyte sintered body satisfies the following formula 1: /()≤ Formula where Ris an intragranular resistance value that is an ion conductivity resistance inside the crystal particles, and Ris a grain boundary resistance value that is an ion conductivity resistance of the grain boundaries between the crystal particles.