Audi will aim for electric cars to account for a quarter of its sales by 2025 as part of a strategic overhaul following the emissions scandal at parent Volkswagen, company sources said, in a move that could step up the challenge to U.S. group Tesla. Audi, which has been slow to embrace battery-powered vehicles, will now invest about a third of its research and development (R&D) budget into electric cars, digital services, and autonomous driving,…