Following the Boeing 787  Li-ion battery incidents of January 2013, this writer has provided comments intended as assistance between experts in neutron radiography, batteries, and safety inspectors. This is to provide an update and overview.

 

The question asked has been whether neutron diagnostic options are being fully considered by those concerned. The emphasis is on “consideration”, not yet actual usage.

 

The considerations suggested would require knowledge and understanding of the diversity of neutron inspection methods developed by many researchers during fifty years. Most of these neutron technique studies are not online. The diversity is such that neither the safety inspectors, nor the manufacturers, are likely to have employees with the ideal depth of understanding in neutron radiology. Therefore this forum may be helpful.

 

In recent days the US battery inspectors have announced that the root cause of the fires remains unclear, but that short circuiting internally in a single cell of the 8 cell battery was suspected. Two suggestions have been put forward by outside observers. One, that during manufacture and assembly faults were incorporated in the separator layer between the metallic anode and cathode sheets. Two, that thin metallic protrusions, known as dendrites, might have grown causing a short circuit between anode and cathode. Presumably another scenario is that a combination of separator inconsistency and lithium dendrites might be the root cause.

 

This leads to the questions of whether there are neutron methods that might contribute to the present investigation or to the planning of future quality control. An observation is that both the hydrogen in the polymer separator layer and the lithium in suspected dendrites are highly neutron attenuating. Could neutron techniques therefore offer information not available using X-rays or other Non-Destructive Inspection (NDI)?

 

This leads to the question what could NDI offer that is not available from destructive tests? That question is best considered by battery experts and investigators currently involved. Of the two burned batteries it seems probable that the cell that triggered the runaway has been destroyed by fire or by destructive inspection. However, the two airlines in Japan together with United Airlines in the USA have reported an unusually high number of batteries needed replacing (over 14).  The unburned cells would almost certainly be too neutron opaque to provide useful neutron inspections even with fast neutrons. However, if sections of the cell contents were carefully extracted, a question could neutron tests provide sensitivity to faults in the separator layer or to lithium metal dendrites, before disturbance by separation of the anode and cathode sheets?

 

Consideration of neutron diagnostic options in this context would include review of available neutron energy spectrums, filters, beam collimations, and imaging systems to provide the highest geometric resolution and contrast sensitivity. Various thermal neutron energy spectrums are available at different centers.  Cold neutron beams and other select energies should also be considered. Collimation ratios as high as 6000 in one axis are available using a slit beam, such as at NIST. Amongst imaging systems, finest grain films (including some used in astronomy) with the finest converter screens such as ultra-thin gadolinium should be compared to alternatives. Is the low energy electron ionizing radiation of the neutron gadolinium reaction able to provide significantly superior geometric resolution than the longer path reactions in scintillators such as LiF-ZnS (Ag)? Track etch imaging as developed for precision neutron radiography in the nuclear fuel tests should be considered. If enhanced discrimination against scattered neutrons is needed, anti-scatter grids should be considered such as those proposed in recent publications by A. Tremsin.

 

In summary it appears that discussions could be valuable particularly between experts in battery technologies and experts in neutron diagnostics. For understandable reasons those with responsibilities within the manufacturers such as Boeing, and the US Safety Inspectors, may be unable to join this discussion in any symmetrical way. However, that should not prevent such discussion being held between others in open website forums. The discussions would be available to any interested party. Outcomes considered particularly valuable could be actively brought to the attention of the concerned parties.

 

This writer recognizes that the root cause of the battery failures may be beyond certain identification and corrective action. Also the aircraft manufacturers and regulators may decide, at least in the near term, to revert to more proven battery types. However, the automobile industry and other users have reportedly been planning to use Li-ion batteries to expand from an industry of about 2 billion US dollars at present, to ten times that within the next few years. If this is true needs for full consideration of neutron test options for Li-ion battery development should remain.

 

Reference.

 

[1] Adame Same, Vincent Battaglia, Hong-Yue Tang, Jae Wan Park. In situ neutron radiography analysis of graphite/ NCA lithium-ion battery during overcharge. J.Appl. Electrochem DOI 10.1007/s10800-001-0363-3. Published online: 19 Nov 2011

 

Li-ion batteries suspected short circuits

Comments by J. P. Barton Feb 19 2013