Currently, most lithium-ion batteries use either graphite or lithium titanate as anodes. The former is extremely energy dense, but can explode or catch on fire if charged too quickly. The latter is safer for rapid charging, but there’s a decrease in energy density which means it needs to be recharged more frequently.
That’s a frustrating trade-off for all sorts of applications, from smartphones to electric cars. Now, researchers at the University of California, San Diego, have discovered a new anode material that could allow batteries to be safely recharged within minutes.
The anode is made up of a substance known as disordered rock salt, made up of lithium, vanadium and oxygen atoms arranged in a similar way to table salt. This new disordered rock salt anode – Li3V2O5 – sits in an important middle ground: it is safer to use than graphite, yet offers a battery with at least 71% more energy than lithium titanate.
“The capacity and energy will be a little bit lower than graphite, but it's faster, safer and has a longer life. It has a much lower voltage and therefore much improved energy density over current commercialised fast charging lithium-titanate anodes," says Haodong Liu, first author of the paper. "So with this material we can make fast-charging, safe batteries with a long life, without sacrificing too much energy density.”
The researchers have formed a company, Tyfast, co commercialise their discovery – aiming first at electric buses and power tools, as charging can be easily scheduled which suits the characteristics of the new material. "For a long time, the battery community has been looking for an anode material operating at a potential just above graphite to enable safe, fast charging lithium-ion batteries. This material fills an important knowledge and application gap," says Ping Liu, whose lab conducted the research. "We are excited for its commercial potential since the material can be a drop-in solution for today's lithium-ion battery manufacturing process.”
The breakthrough is the culmination of six years of research into the potential of disordered rock salt – although it was first tested as a cathode. Liu says the UC San Diego team decided to test the material as an anode based on a hunch.
“When people use it as a cathode they have to discharge the material to 1.5 volts," he says. "But when we looked at the structure of the cathode material at 1.5 volts, we thought this material has a special structure that may be able to host more lithium ions – that means it can go to even lower voltage to work as an anode.”
In the study, the team found that their disordered rock salt anode could reversibly cycle two lithium ions at an average voltage of 0.6 V – higher than the 0.1 V of graphite, eliminating lithium metal plating at a high charge rate which makes the battery safer, but lower than the 1.5 V at which lithium-titanate intercalates lithium, and therefore storing much more energy.
The researchers showed that the Li3V2O5 anode can be cycled for over 6,000 cycles with negligible capacity decay, and can charge and discharge energy rapidly, delivering over 40 percent of its capacity in 20 seconds. The low voltage and high rate of energy transfer are due to a unique redistributive lithium intercalation mechanism with low energy barriers.
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