In search of higher energy densities, dendrite growth is a significant concern, especially for next-generation lithium-ion batteries that use lithium metal for primary construction of both electrodes (as opposed to one carbon-based electrode). In the Eaton 5P lithium-ion UPSs, the lithium-ion cells use a Lithium Iron Phosphate (LiFePO4, or LFP) positive electrode and a graphite (carbon) based negative electrode, instead of a lithium metal-based negative electrode that is more-susceptible to lithium dendritic growth1-6. Moreover, LFP cells are one of the safest lithium-ion chemistries widely available today due to the high thermal runaway temperature and tolerance to voltage variation and abuse7.
In charged 5P cells, intercalated lithium ions flow from the carbon-based negative electrode (anode during discharge) to the positive LFP electrode (cathode during discharge); the ratio and structure of carbon atoms to lithium atoms in the negative electrode at various states of charge capacity help prevent large dendrites from growing on the carbon surface when operated within designed voltage ranges1,2,3, which is carefully controlled by the onboard BMS. Though the cells used in the 5P lithium-ion UPSs are relatively expensive, they are designed to support the high discharge rates to which they can be subjected in this application while emphasizing safety in both construction, material and chemistry.
During charging in the 5P lithium-ion UPS, a relatively low maximum charge current of 0.25C is used (the cells are rated for a maximum charge rate of 1°C nominal, 2°C max), which helps to mitigate tree-like dendrites from growing on the positive LFP electrode when lithium ions flow back to the carbon-based electrode3. Even at this low C-rate for charging, the 5P lithium-ion UPS typically requires only six to eight hours to reach full charge, depending on the previous rate of discharge while supporting load on battery.
The LFP cells used in the 5P lithium-ion UPS are of high quality, with manufacturing processes in place to reduce impurities and moisture while continually monitoring quality on the production line. Each cell includes built-in over charge protection, pressure vent, anti-puncture ceramic coating and a barrier on the positive LFP electrode to help mitigate short circuiting from any dendritic growth.
Charge at low ambient temperatures – The BMS does not allow charging below 0°C by turning off the charger at this temperature. The UPS is rated for 5 to 40°C ambient temperatures, and self-heating for cells during discharge at various loads is fully accounted.
Over and under temperature protection – During charge, discharge and normal operation, the BMS turns off if one of the multiple pack temperature monitoring points is too high or too low for the specific state of operation (as both charging and discharging allow different minimum and maximum temperature thresholds for the cells).
Charging at an excessive rate – In the 5P lithium-ion UPS, the cells charge at a maximum rate significantly below the 1°C nominal rate of the 2500 mAh cells due to the 12S2P configuration and the UPS charger capability – significantly below the allowable cell charge rate. The cells are rated for up to a 1°C nominal charge rate, with a 2°C maximum charge rate.
Overcharge protection – The BMS limits every cell to a maximum charging voltage of 3.6V, and 43.2V for the battery pack. These limits keep the cell voltage below the maximum design limit.
Overdischarge protection – The BMS limits the minimum voltage of each cell and the battery pack to prevent damage.
Overcurrent protection – The BMS limits the maximum charging and discharging current to remain within tolerances. Moreover, the UPS charger is also limited in current provided to the battery pack. In combination with cell voltage monitoring and various temperature monitoring points, the BMS prevents thermal runaway due to excessive charge or discharge currents. Moreover, the cells used in the 5P lithium-ion UPS are capable of discharge currents beyond our limit.
Floating charge protection – The BMS further limits charging current if near the maximum cell voltage level.
Cell-to-cell voltage variation and balancing – The BMS continually monitors cell to cell voltage variation and will shut down the pack if a cell strays out of tolerance. This also includes cases where voltage is reduced on a cell due to higher currents from reduced internal resistance.
Capacity decay – The BMS monitors various factors to predict capacity decay.
The 26650 form-factor cells in the Eaton 5P 1U lithium-ion UPS utilize a type of lithium-ion chemistry known as Lithium Iron Phosphate (LiFePO4, or LFP) for the positive electrode. Since there is enough physical space available in this 1U UPS application to meet runtime targets using cells with one carbon-based (negative) electrode, an extremely high energy density cell using lithium metal for both electrodes is not needed1-6.
Furthermore, LFP lithium-ion chemistry is more-tolerant to variations in voltage and has a much higher thermal runaway point (~270°C) than the Lithium Cobalt Oxide (LCO; 150°C) chemistry first commonly used in consumer electronics like cell phones and laptops7. Moreover, LFP offers similar benefits over other popular lithium-ion chemistries such as Nickel Cobalt Aluminum Oxide (NCA; ~150°C), Lithium Manganese Oxide (LMO; ~250°C) or Lithium Nickel Manganese Cobalt Oxide (NMC; ~210°C) found in power tools, some UPS, and many electric vehicles to date7. Lithium-ion batteries using LFP chemistry also have a strong Fe-P-O bond that resists redox reactions better than Cobalt-based lithium-ion batteries due to external conditions like short-circuit, overheating, etc.8
Our lithium-ion cells and battery pack in the 5P lithium-ion UPS comply with both UL1642 and UL1973, as well as the DOT shipping standard UN/DOT38.3 and IEC 62133.
Between UL1642 and UL1973, the following list of safety tests are performed on the lithium-ion cells and/or battery pack:
As part of DOT/UN38.3 testing, the case temperature of cells may not exceed 170°C; since the LFP cells used in the 5P 1U lithium-ion UPS have a thermal runaway point much greater than this temperature, risk of thermal runaway when exposed to situations like the test conditions is further reduced.
Lastly, Eaton has conducted rigorous internal cycle and performance testing of the 5P 1U lithium-ion UPS to characterize safety and performance over a multi-year period prior to launch. While supporting full output load at 40°C (104°F), which is the maximum ambient temperature rating of the UPS, test units were discharged and charged continuously (approximately 3 cycles per day) until all samples reached our cycle life targets. In all cases using our 26650-based battery packs, all tested units met our cycle life targets with zero pack failures or issues. Since quality, safety, and reliability are of utmost concern, we continue to test and evaluate our lithium-ion UPS into the future.
1 Rensselaer Polytechnic Institute. “Temperature heals lithium dendrites.” TechXplore. 03/30/2018, https://techxplore.com/news/2018-03-temperature-lithium-dendrites.html. Accessed 5 June 2019.
2 Clemens, Kevin. “Three Ways That Lithium Dendrites Grow.” Design News. 11/05/2018, https://www.designnews.com/electronics-test/three-ways-lithium-dendrites-grow/78500767259733. Accessed 22 May 2019.
3 Clemens, Kevin. “Heating Heals Lithium Dendrites.” Design News. 04/24/2018, https://www.designnews.com/electronics-test/heating-heals-lithium-dendrites/211268355358591. Accessed 6/5/2019.
4 Yarris, Lynn. “Roots of the Lithium Battery Problem: Berkeley lab Researchers Find Dendrites Start Below the Surface.” Science Shorts. 12/17/2013, https://newscenter.lbl.gov/2013/12/17/roots-of-the-lithium-battery/. Accessed 24 May 2019.
5 Jefferson, Brandie. “Whiskers, surface growth and dendrites in lithium batteries.” Washington University in St. Louis. Phys.org. 10/25/2018, https://phys.org/news/2018-10-whiskers-surface-growth-dendrites-lithium.html. Accessed 24 May 2019.
6 Wood, Kevin N. et. al. “Dendrites and Pits: Untangling the Complex Behavior of Lithium Metal Anodes through Operando Video Microscopy.” ACS Cent. Sci. 2016, 2, 11, 790-801. Publication Date: October 14, 2016, https://doi.org/10.1021/acscentsci.6b00260. Accessed 24 May 2019.
7 Battery University. “BU-205: Types of Lithium-ion.” Battery University. Last updated 04-24-2019, https://batteryuniversity.com/learn/article/types_of_lithium_ion. Accessed 24 May 2019.
8 “Harding Energy | Lithium Ion batteries | Lithium Polymer | Lithium Iron Phosphate". Harding Energy. http://www.hardingenergy.com/lithium-2/#phosphate. Accessed 6 June 2019.