Learn Spanish (Latin America): Este teléfono tiene una mejor batería.

Usually, descriptive adjectives in Spanish go after the noun. However, the irregular comparatives mejor (better) and peor (worse) are exceptions. They almost always go before the noun they describe, just like in English.

Does mejor change depending on the gender of the noun?

No, mejor stays the same for both masculine and feminine nouns. That is why we say una mejor batería (feminine) and un mejor teléfono (masculine). It only changes for plural nouns, where it becomes mejores.

Why is it este teléfono and not esto teléfono?

Este is the masculine singular word for 'this' when it comes directly before a masculine noun like teléfono. Esto is the neutral version of 'this,' and it is only used when referring to an unknown object, a statement, or a general idea, never right before a specific noun.

Could I say una batería más buena instead of una mejor batería?

While people might understand you, it sounds very unnatural. Just like in English where we say 'better' instead of 'more good', Spanish uses the irregular comparative mejor instead of más buena.