Electric vehicles could make up half of all new car sales by 2040, as long as oil prices eventually increase, according to a study released Thursday by Bloomberg New Energy Finance.  The cost of the lithium-ion batteries that power electric vehicles is quickly decreasing, but sales suffered last year as oil slid below $30 per barrel, making gas-guzzling vehicles more affordable in the United States. If oil remains at rock-bottom prices, it could delay electric vehicles from becoming mainstream for at least the next four years. “In a scenario where they become widespread in fleets and ride sharing scheme, new EV sales could reach 50% of new car sales by 2040,” Salim Morsy, senior analyst at BNEF, wrote in the study. “However, persistently low crude oil prices could also keep adoption as low as 25% by 2040.”