LG Chem Ltd., the world's largest lithium-ion battery maker, said Monday its net profit declined 39.5 percent in the fourth quarter from a year ago due to a prolonged economic downturn in the industry and a stronger won. Net profit came to 176.88 billion won (US$163.3 million) in the October-December period, compared with 292.32 billion won a year ago, LG Chem said in a regulatory filing.