Digipower Nikon battery grip. (PGR-NKD31) It says its for D3200/D3300/D5200/D5300 but I checked online it says its for the D3100 too.
http://www.bestbuy.ca/en-CA/product/digipower-digipower-nikon-d3200-d3300-d5200-d5300-battery-grip-pgr-nkd31-pgr-nkd31/10176368.aspx
Read more: Will this Battery Grip work on my D3100?