TOKYO -- Lithium-ion batteries are in everything -- smartphones, wearables, drones, electric vehicles, power system stabilizers, even jetliners.

But safer, cheaper and higher-capacity power packs are poised to replace them.

While still years away, post-Li-ion batteries are no longer stuck in the distant future.

This past year has seen a number of significant technological breakthroughs. If only one of a number of promising technologies can be commercialized, we can expect dramatic performance improvements from a host of gadgets and machines.

One alternative is the solid-state battery, which uses a solid electrolyte instead of the electrolytic solution that does the heavy lifting in transporting the positive lithium ions between the cathode and anode in today's batteries. Researchers have succeeded in developing an efficient electrolytic solid material that significantly improves lithium ion conductance, raising hopes that batteries with much higher power densities are edging closer to reality.

Lithium-air batteries, meanwhile, have the ability to greatly improve energy density, but researchers have hit a discharge wall -- these power packs can't put out much current for very long. Yet this technology could also be on the cusp of a dramatic turn: Researchers have succeeded in raising the density close to theoretically expected levels, if only for a single charge cycle.

The need to compromise

Another breakthrough battery does not use lithium. Researchers have succeeded in creating a cathode material for the sodium-ion battery; its discharge capacity beats that of Li-ion cells. The material also enables the power packs to be recharged upward of 500 times, overcoming one of the weaknesses limiting the technology's prospects.

Battery research has undergone a big shift in recent years. It used to be that nearly half of the presentations at the Battery Symposium in Japan were about fuel cells and Li-ion battery cathode materials. But between 2012 and 2016, the number of fuel cell presentations dropped in half; those regarding cathode materials have decreased by a third since 2012. Presentations on solid-state, lithium-air and non-Li-ion batteries increased by 50% to 100% over the same period.

Toyota Motor in recent years has been a big sponsor of the Battery Symposium in Japan. The automaker has focused on developing solid-state and Li-air batteries. At the symposium in November, the automaker discussed a scenario for transitioning from Li-ion batteries to solid-state and then Li-air batteries in its vehicles.

"We want our electric cars to go 500km" on a single charge, said Shinji Nakanishi, a battery researcher at Toyota. "And for this, we want rechargeable batteries that can generate 800 to 1,000 watt-hours per liter." That is two to three times the energy density that today's best Li-ion batteries can pack.

These breakthroughs are not coming a day too soon. For decades, consumers, consumer electronics makers and other power-hungry industries have been clamoring for batteries that last longer between charges. Rechargable batteries that are more forgiving to slight manufacturing imperfections would also be nice. Just ask South Korea's Samsung Electronics, which suffered huge losses after batteries in some of its flagship Galaxy Note 7 smartphones burst into flames.

The scandal provided a stark lesson to makers: No matter what type, batteries store large amounts of energy that can combust if they are defective. Fearing similar risks, producers have shifted their focus to safety, even if doing so means compromising on energy density, or the battery's capacity.

Li-ion's fast-approaching ceiling

At Panasonic, which supplies huge quantities of Li-ion power packs to U.S. electric vehicle maker Tesla Motors, safety concerns have the electronics maker taking a long look at a new technology.

"We think the existing technology can still extend the energy density of LIBs by 20% to 30%" President Kazuhiro Tsuga said. "But there is a trade-off between energy density and safety. So if you look for even more density, you have to think about additional safety technology as well. Solid-state batteries are one [possible] answer."

For decades now we have been pushing the limits of our Li-ion batteries in terms of energy density. Today's best Li-ion cells can put out about 300 watts per kilogram; a package of Li-ion cells can give off from 150 watts to 250 watts per kilogram. These levels are already close to the theoretical maximum, said Naoaki Yabuuchi, an associate professor at Tokyo Denki University.

"Existing LIBs still have room to improve their energy density because you can raise the density by introducing a nickel-based cathode material so you can expect the batteries will still be used in the next few years," said Yabuuchi, an expert on various types of rechargeable batteries.

Yabuuchi mentioned two nickel-based cathode materials -- lithium nickel cobalt aluminum oxide and lithium nickel manganese cobalt oxide.

But when it comes to what technologies might be commonplace five to 10 years from now, none have a clear potential for practical use, Yabuuchi added. In his view, Li-ion batteries will reach a ceiling as early as the first half of 2020 -- if their development continues to rely on existing technologies.

Sodium, magnesium and zinc

Yet another of Li-ion limitation is cost; prices for cathode materials are soaring and are expected to remain high. Prices of lithium carbonate, for example, rose threefold from October 2015 to March 2016. They have not dropped any appreciable amount since. Prices of cobalt, the most expensive of the materials, have been on their own dramatic climb during the past year.

Prices are rising as demand for the materials balloons. China is on an electric car and bus binge, and if Tesla makes and sells as many electric vehicles as it thinks it can, it would exhaust the world's current annual production of Li-ion batteries. So it's getting ready to make its own batteries at a plant called the Gigafactory, in the U.S. state of Nevada.

Industry analysts say supplies of lithium and cobalt are now getting so tight that their scarcity could severely limit the production of electric vehicles.

It may seem paradoxical, then, that Li-ion battery prices have significantly fallen. A few years ago, it would take 200,000 yen (about $1,800 today) to buy 1 kilowatt-hour of Li-ion capacity. Now about 30,000 yen buys the same amount of capacity.

The reduction comes from manufacturers cutting production costs by bringing much more efficient factories on line. While producers have come through with cost-saving factories, they also began needing more materials. Now their materials prices wildly fluctuate. For vehicle-use Li-ion batteries, costs for cathode materials like cobalt and lithium represent nearly a third of the overall cell price.

That makes lithium alternatives like sodium, magnesium and zinc attractive alternatives. Deposits of these minerals remain abundant, so the materials don't cost so much. And as they get produced in larger quantities, their prices will be less prone to wild fluctuations -- even if the demand for electric vehicles spikes.