MILAN, June 19 (Reuters) - Automaker Stellantis holds a 9.5% stake in U.S. solid-state battery startup Factorial Energy ( FAC ) and might buy further shares in the future, a SEC filing showed.

Solid-state batteries for electric vehicles are expected to be lighter, with better energy density than, traditional lithium-ion batteries, and ensuring faster charging times. However manufacturers have so far struggled to produce them at scale.

-- Stellantis' ( STLA ) overall stake in Factorial is valued around $126 million at current stock prices.

-- Stellantis ( STLA ) sees Factorial as an "attractive investment opportunity", the filing showed.

-- The Franco Italian automaker first invested in Factorial in 2021, with a €75 million ($86 million) deal.

-- Stellantis ( STLA ) said earlier this month it had integrated Factorial's 'FEST' solid-state battery cells into a development vehicle and launched a road-testing program to verify performance, safety and reliability.

-- According to the filing, Stellantis' ( STLA ) Financial Services CEO Jon Nelson has been elected to Factorial's board.

($1 = 0.8728 euros)