Why 2023 was the hottest summer yet, and a way to improve lithium-ion batteries

With the help of tree-ring data, scientists have found summer 2023 was the hottest thus far. Researchers have developed a new electrolyte that improves lithium-ion batteries.

Rings of a redwood tree in California. Tree rings contain annually resolved and absolutely dated information about past summer temperatures. | Photo Credit: Bill Oxford/Getty Images/iStockphoto

2023: The hottest summer yet

SCIENTISTS have determined that 2023 was the hottest summer in the Northern Hemisphere in the past 2000 years, almost 4 degrees warmer than the coldest summer in the same period.

Instrumental evidence only reaches back as far as 1850 at best, and most records are limited to certain regions. Also, according to researchers from the University of Cambridge and the Johannes Gutenberg University Mainz, Germany, the early instrumental records, 1580–1900, are sparse and inconsistent. The scientists compared the early data with a large-scale tree-ring dataset and found that the 19th century baseline used to contextualise global warming is several tenths of a degree colder than previously thought. By recalibrating this baseline, they found that summer 2023 conditions in the Northern Hemisphere were 2.07°C warmer than the mean summer temperatures during 1850-1900. The maximum exceeded the extremes of natural climate variability by half a degree.

The finding, reported in Nature, also demonstrates that the 2015 Paris Agreement to limit warming to 1.5°C above pre-industrial levels has already been breached, at least in the Northern Hemisphere.

“When you look at the long sweep of history, you can see just how dramatic recent global warming is,” said co-author Ulf Büntgen of Cambridge University, “...and this trend will continue unless we reduce greenhouse gas [GHG] emissions dramatically.”

Büntgen questioned the use of the 1850 baseline. “What is normal, in the context of a constantly changing climate, when we’ve only got 150 years of meteorological measurements?” he asked. “Only when we look at climate reconstructions can we better account for natural variability and put recent anthropogenic climate change into context.”

Tree rings can provide that context, since they contain annually resolved and absolutely dated information about past summer temperatures. Using tree-ring chronologies allows researchers to look much further back in time without the uncertainty associated with some early instrumental measurements.

The available tree-ring data revealed that cooler periods over the past 2,000 years, such as the Little Antique Ice Age in the 6th century and the Little Ice Age in the early 19th century, followed large-sulphur-rich volcanic eruptions. Volcanic eruptions spew huge amounts of aerosols into the stratosphere, triggering rapid surface cooling. The coldest summer in this period, in 536 CE, followed one such eruption and was 3.93°C colder than the summer of 2023.

Most of the warmer periods covered by the tree-ring data can be attributed to the effects of the El Niño–Southern Oscillation phenomenon. El Niño affects weather worldwide and often causes warmer summers in the Northern Hemisphere. El Niño events, according to the scientists, can be observed in the tree-ring data much further back in time.

However, over the past 60 years, global warming caused by GHG emissions are causing El Niño events to become stronger, resulting in hotter summers. The current El Niño event is expected to continue into early summer 2024, making it likely that this summer will break temperature records once again. “When you look at the big picture, it shows just how urgent it is that we reduce GHG emissions immediately,” said Jan Esper, the lead author, from Mainz.

The scientists also noted that it was difficult to obtain similar historical averages for the Southern Hemisphere for the same period since data are sparse. The south also responds differently to climate change since it is far more ocean-covered than the north.

Lithium-ion batteries (LIBs) power electric cars, tablets, and smartphones and also store energy in power plants. Here, a close-up of the LIB packs in an electric car. | Photo Credit: Getty Images/iStockphoto

New electrolyte to improve lithium-ion batteries

RESEARCHERS from Ningbo University, China, and the University of Puerto Rico-Rio Piedras Campus, US, have developed an ultralow-concentration electrolyte that greatly improves the production, performance, and recycling of lithium-ion batteries (LIBs), which power electric cars, tablets, and smartphones and also store energy in power plants. The work was reported in a recent issue of the journal Angewandte Chemie.

Lithium salts, while enhancing battery power, often contribute to high costs. An ultralow-concentration electrolyte using the lithium salt lithium difluoro(oxalato)borate (LiDFOB) may offer a more economical and sustainable alternative, the study said. Cells employing these electrolytes and conventional electrodes perform better while also potentially streamlining battery production and recycling processes.

LIBs usually comprise lithium cobalt oxide (LiCoO₂) cathodes, graphite anodes, and liquid electrolytes, which facilitate ion movement between the electrodes. Crucially, electrolytes regulate the formation of the interphase layer—the passivation layer formed on the surface of the anode as a result of electrolyte decomposition—influencing battery cycling efficiency and other performance metrics. Commercial electrolytes, however, continue to primarily rely on a system developed more than 30 years ago: a carbonate solvent, or carboxylic acid esters containing 1.0-1.2 mol/l of lithium hexafluorophosphate (LiPF₆).

Innovations to enhance battery performance have hitherto largely involved increasing the electrolyte concentrations up to 3 mol/l to achieve the formation of robust interphase layers. However, there are issues with these electrolytes, including high viscosity, limited wetting capability, and subpar conductivity. They also require large amounts of lithium salts making them prohibitively expensive.

While electrolytes with concentrations below 0.3 mol/l promise cost reduction, they come with their own problems such as increased solvent decomposition, leading to formation of a less stable interphase layer dominated by organic compounds.

This study found that LiDFOB serves as a cost-effective alternative to the commonly used LiPF₆ additive. Paired with ethylene carbonate-dimethyl carbonate, a standard carbonate solvent, LiDFOB achieves a record-breaking low salt content of just 2 weight per cent (equivalent to 0.16 mol/l). Despite this minimal salt concentration, it exhibits a commendably high ionic conductivity of 4.6 millisiemens/cm, rendering it viable for battery operation.

Furthermore, the anions of DFOB form a robust, inorganic-dominated interphase layer on both  electrodes, which greatly enhances the cycling stability in both half and full cells.

While currently used LiPF₆ decomposes in moist conditions, releasing the highly toxic and corrosive hydrogen fluoride gas, LiDFOB is moisture-resistant and environment-friendly. Instead of strict dry room conditions, LIBs with LiDFOB can be made under ambient conditions, an additional cost-saving feature. Battery assembly and recycling would also be greatly facilitated and lead to more sustainability, said the researchers.