China’s push to promote electric cars comes with a lot of benefits for a country that suffers from terrible air pollution from its reliance on fossil fuels. But there’s always a downside—electric car batteries are toxic if not disposed of properly, and China’s on the verge of having to deal with a slew of batteries that can no longer hold a charge.

In just a few years, China has become the world’s biggest electric vehicle market, with the help of subsidies. It saw 336,00 new electric car registrations (pdf, p.12) in 2016, according to the International Energy Agency. That includes both battery-only and hybrid models. Including other types of vehicles, China says it sold a total of half a million “new energy” vehicles last year. This month, China also said that it would eventually phase out sales of all fossil-fuel cars.

That fast-growing market, however, is also producing batteries at a faster rate too. The average lifespan of a lithium-iron phosphate (LFP) battery, the dominant type in China’s electric vehicles, is around five years, according to Li Changdong, chairman of the Hunan-based Brunp group, China’s top electric car battery recycler in 2016 (link in Chinese). Most batteries installed on electric vehicles during the 2012 to 2014 period will be retired on a large scale (link in Chinese) around 2018, Li told the Beijing-based newspaper Economic Information Daily.

In 2020, nearly 250,000 metric tons (276,000 tons) of batteries, or 35 gigawatt-hours of batteries, are set to be retired—nearly 20 times those depleted in 2016, GaoGong Industry Institute, a Shenzhen-based electric car industry research firm, told Quartz.

The battery is the heart of the electric vehicle industry, and the country needs a well-established battery recycling system, Xin Guobin, a top industry and tech official, told a national forum for the battery-powered engine industry Tuesday (link in Chinese) (Sept. 26). But recycling these batteries isn’t easy, due to the sophisticated chemical procedures involved. If it’s not done properly the heavy metal contained in the battery can lead to contamination of soil and water.

According to China’s 2015 electric vehicle battery policy, car manufacturers are responsible for recycling their batteries (link in Chinese). But many auto manufacturers often leave the responsibility to battery suppliers, who find it hard to afford the cost of building a national recycling network, noted the Economic Information Daily. For now, China’s battery recycling industry is relatively small and scattered, and recycling operating costs are high, Gao Xiaobing, director of the lithium battery study center at GaoGong, told Quartz. That’s discouraging more players from entering the business.

China’s not the only one facing a recycling headache. In the European Union, only 5% (pdf) of lithium-ion batteries, another common type of battery power used in electric vehicles are recycled, according to data from non-governmental environment advocacy group Friends of the Earth Europe, which pointed out that “most of the current lithium is either dumped in landfill or incinerated.”