US electric vehicle OEM Tesla’s latest patent suggests it has solved the failure issues of high operating temperatures and voltages in lithium-ion technology to make a battery that will last a ‘million miles’.
Tesla’s lead battery researcher Jeff Dahn and members of the Department of Physics and Atmospheric Science in Dalhousie University suggest their lithium-ion pouch cells have a 6,000 cycle life, or 20 years in grid energy storage applications.
The results were published in the Journal of the Electrochemical Society.
Researchers used a single crystalline structure in conjunction with the mix of ‘secret sauce’ additives in the electrolyte— although the report says these could be ‘application specific’.
Single crystalline structures are more robust and reduce grain boundaries that may provide better electrolyte stability at high voltages, lithium-ion expert Chris Hale, from Upgradetech Engineers, told BEST Battery Briefing (see his full analysis below).
However, Hale said: “Until I can hold in my hand a commercially produced cell capable of several thousand cycles, I will always remain sceptical.
“At the end of the day, the research on ‘secret sauce’ additives has been going on for years and it’s been a couple of years since the sinle crystalline work was carried out, I would have thought that we may have seen commercially available cells promoted to have much better cycle life coming through by now.
“It will be interesting to see what LG-Chem come up with in the next year or so.”
The breakthrough follows three years of testing, which include long-term cycling at 20, 40 and 55°C, long-term storage at 20, 40 and 55°C, and high precision coulometry at 40°C.
The paper notes that several different electrolytes were considered in this LiNi0.5Mn0.3Co0.2O2/graphite chemistry, including those that can promote fast charging.
Tesla uses cylindrical 18650 cells from Panasonic, and last announced plans for 2170 cells for its Model 3 packs— and may move to NMC technology for the Chinese version of the Model 3.
Analysis by lithium-ion expert Chris Hale, from Upgradetech Engineers
Firstly, its Tesla – they are not renowned for making wild claims, they would have funded the research, which has obviously taken at least two-three years to accumulate, and this is their output.
If we dig a bit deeper, Jeff Dahn (key author) discussed key cell failure mechanisms and how to improve them in his 2013 video lectures ‘How do Li-Ion batteries die, and how to improve the situation’.
In a nutshell, there are a number of factors that effect ageing of a cell (see below), the most prominent means of promoting accelerated ageing are; temperature and high voltage, the two key components that Dahn has lectured on in the past and clearly topics he has targeted for his research.
The report has indicated that longevity can be maintained at elevated temperature and high voltage, so it would seem to suggest that they have cracked a couple of the key failure mechanisms. To achieve this it is necessary to look at the factors that promote cell ageing:
Briefly:
- Temperature – increases the parasitic reactions (between electrode and electrolyte).
- High voltage – promotes electrolyte-oxidation products that lead to a gradual build up of by-products on the negative electrode until the graphite pores become blocked, leading to lithium plating and rapid capacity loss. Also dissolution of transition metals, among other side reactions.
- Loss of active material – Lithium being consumed in SEI.
- Growth of SEI layer (consumed Transition metals) – increasing cell impedance.
- Dendrite growth – accumulated lithium deposited on electrodes during cycling.
- Decomposition of electrolyte.
Secondly, Dahn, a professor at Dalhousie University, has authored a number of papers on cell longevity. Tesla has chosen the Dalhousie University as a partner for cell research. Interestingly, the fact that Tesla has reportedly got a longevity program also lends credibility to the report.
Thirdly, Tesla has patented this technology. The patent describes what has been covered in the article. There are in fact two articles, the second being ‘Comparison of Single Crystal and Polycrystalline LiNi0.5Mn0.3Co0.2O2 Positive Electrode Materials for High Voltage Li-Ion Cells’ (published in 2017).
The single crystalline structure in conjunction with the mix of ‘secret sauce’ additives could certainly have the potential of increasing cell longevity.
Single crystalline structures have the advantage of being more robust, reduced grain boundaries (defects that reduce electrical properties). That may provide better electrolyte stability at high voltages.
Additive mixes have the advantages (among other things) of reducing the electrolyte oxidation at the positive electrode and potentially reducing the formation of solid products on the negative electrode.
Fourthly, Tesla have recently engaged in a deal to build a gigafactory in China, producing NMC811 (LG Chem) cells for their automotive program, replacing the NCA cell’s they have traditionally used for automotive application.
(Note: although the article references NMC532, the latest research has put NMC811 as a targeted next-level cell with higher energy density, the technology for the 532 can be used in the 811).
NCA was traditionally used over NMC because of longer driving range, however NMC811, especially with enhanced cycle life, now seems to be the preferred option.
