Thank you for your question about using coffee grounds on your peace lily and how to keep leaves from browning.  

Coffee grounds can be an excellent soil amendment in the outdoor garden because of their moderate fertilization properties and the organic matter they add to the soil.  However, care must be taken when using on houseplants.  Coffee grounds are acidic, and, while most houseplants prefer slightly acidic soil regular use of the grounds can lower the pH to unacceptable levels.  Also, peace lilies benefit from a light fertilizer application a couple of times a year, so the use of the nitrogen containing coffee grounds can result in excessive fertilization, which weakens the plant.

Browning leaves can be caused by light levels and excessive or insufficient water.  Peace lilies prefer bright, indirect light.  Not enough light and the leaves will turn a light shade of green.  With excessive light they will sunburn---turning yellow and brown.  

Peace lilies prefer evenly moist soils.  When there is loo little water leaves turn yellow at the margins, with the yellow moving towards the center of the leaf, and then turning brown.  Too much water results in oxygen starvation and the leaves also turn yellow and then brown.

Because the peace lily requires little fertilization it is not likely that the browning of your leaves is caused by a lack of nitrogen. 

Here's an excellent article on growing peace lilies from the University of Vermont.    https://pss.uvm.edu/ppp/articles/peacelilies.html

Good luck with your peace lily.