The Berkeley Lab team was led by Richard Saykally, a chemist with Berkeley Labs Chemical Sciences Division, David Prendergast, a theorist with Berkeley Labs Molecular Foundry, and Steven Harris, a chemist with the Labs Materials Sciences Division.
A crucial process in lithium ion batteries is the transport of lithium ions between the electrodes, explained Saykally. Commercial lithium-ion batteries contain a liquid electrolyte comprising a lithium salt dissolved in an alkyl carbonate solvent system. Theres disagreement in the battery industry on the nature of the local solvation environment of lithium ions in these solutions, a critical issue because the desolvation of the ions as they move through the negative electrode is believed to limit the electrical power that can be made available.
Most previous computational simulations have predicted a tetrahedral solvation structure for the lithium ion in the electrolyte, but the new study by Saykally, Prendergast, Harris and their collaborators show this to not be the case.
Our results indicate a solvation number of 4.5, which points to a non-tetrahedral solvation structure for the lithium ions, explained lithium-battery expert Harris. This contradicts numerous theoretical studies which indicated a primarily tetrahedral coordination structure with a solvation number near 2 or 3, depending on the prevalence of ion pairing. Based on our results, to design better performing electrolytes, future computational models will need to move beyond tetrahedral coordination structures.
Lithium-ion batteries (LIBs) have shortcomings high costs, inadequate energy densities, long recharge times and short cycle-life times have hampered the use of LIBS for electric vehicles and for efficient electrical energy storage systems that can be used in conjunction with wind and solar energy sources.
Although it has become clear to the battery industry that improvements in the liquid electrolyte are essential if LIBs are to be effective for electric vehicles and large-scale energy storage, most LIB research has focused on the electrodes and solid electrolyte interphase. The problem has been a lack of capabilities for the requisite experiments, particularly X-ray spectroscopy.
The deficiency was addressed by Saykally and his group with their development of a liquid microjet technology in which two aqueous samples rapidly mix and flow through a finely tipped silica nozzle only a few micrometers in diameter. The resulting liquid beam travels a few centimeters in a vacuum chamber before it is intersected by an X-ray beam then collected and condensed out. This liquid microjet system has been set up at Beamline 8.0.1 of Berkeley Labs Advanced Light Source (ALS). Beamline 8.0.1 is a high flux undulator beamline that produces X-ray beams optimized for X-ray spectroscopy.
Working at the ALS with our liquid microjet system, we used X-ray absorption spectroscopy to study lithium tetrafluoroborate in propylene carbonate, explained Saykally. X-ray absorption spectroscopy is an atom-specific core-level spectroscopic probe of unoccupied electronic states. It is highly sensitive to both the intra- and intermolecular environment of the target atom.
The XAS experimental spectra were interpreted through molecular dynamics and density functional theory spectral simulations carried out on the supercomputers at the National Energy Research Scientific Computing Center (NERSC) by Prendergast and Jacob Smith, a graduate student in Saykallys research group. The ALS, the Molecular Foundry and NERSC are all DOE Office of Science national user facilities hosted at Berkeley Lab.
A paper entitled ‘X-Ray absorption spectroscopy of LiBF4 in propylene carbonate: a model lithium ion battery electrolyte’ has been published in the journal Physical Chemistry Chemical Physics.
Source: http://www.electronics-eetimes.com/en/berkeley-lab-pinpoints-better-electrolyte-for-lithium-ion-batteries.html?cmp_id=7&news_id=222923392&vID=209
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