A team a the University of Warwick has fully characterized the thermal behavior of a large-format 20 Ah lithium iron phosphate pouch cell over a wide range of ambient temperatures and C rates during both charging and discharging.
The findings, reported in the Journal of Power Sources, are intended to improve understanding of localized aging of the cell—and thus premature aging of battery packs. The work also provides insight into creating a guideline for instrumentation and determining where to place thermal sensors, as well as giving information on the necessary cooling strategy. The researchers suggest that improved temperature sensing methodology could underpin the development for more efficient battery thermal management systems, reducing computational complexity, weight and cost.
Key Li-ion battery characteristics such as capacity and impedance worsen due to aging mechanisms such as solid electrolyte interphase (SEI) layer growth. Increased impedance is detrimental as less power can be extracted from the cell and additional heat is generated. Cooling systems need to extract efficiently the extra heat produced.
Experimental work has shown that the aging rate has a strong temperature dependence. Operating cells at elevated temperatures (>25 ˚C) accelerates SEI film growth on the anode and degrades of the cathode, leading to capacity fade and increased internal impedance. The latter produces more heat and further accelerates aging in a feedback loop. The increased heat generation adds extra cooling requirements to the cooling system and may have catastrophic consequences such as thermal runaway if the cell temperature cannot be managed to an appropriate level throughout battery life for a range of different environmental conditions and use cases, the researchers noted in their paper.
Significant thermal gradients may develop inside lithium-ion batteries during charging and discharging, which are more pronounced with increasing cell size and current rate. Thermal gradients pose a significant challenge in lithium ion battery pack design, where it is crucial to maintain temperature homogeneity between the cells that constitute the battery pack in order to prevent adverse voltage distributions and differential ageing between the cells. Consequently, manufacturers dedicate a large amount of resources to develop battery packs with corresponding thermal management systems, which aim to maintain each cell in identical thermal operating conditions. These thermal management systems require accurate temperature sensing and comprehensive instrumentation of the battery system. However, commercial battery packs commonly have limited numbers of thermocouples to reduce cost, weight and computational complexity.
As a consequence of thermal gradients, the cell does not age uniformly and an ageing gradient occurs inside the cell, reducing the efficiency and lifespan of lithium ion batteries. Strong thermal gradients can also lead to deformations of the jelly roll in cylindrical cells. The geometric orientation of the temperature gradient is also important: for a pouch cell, it has been shown that temperature gradients perpendicular to the stack layers lead to higher local currents and faster degradation compared to temper- ature gradients in-plane with the stack layers.
… Large pouch cells are currently used in production vehicles such as the Nissan Leaf, Renault Zoe, and Daimler Smart. The battery packs of EVs and HEVs deliver high power demand and accept high charge power at a wide temperature range from sub-zero to 40 ˚C ambient conditions, and premature aging of the battery packs has been reported in main-stream news media. The root cause of this premature aging could be the temperature gradient, originate from high power demand. Despite the existing literature as mentioned earlier, indicating the persistence of temperature gradient and its implications on ageing and safety, a comprehensive study on large format cells with a wide range of temperatures and charge-discharge rates, matching with automotive application is currently not present in the literature.—Grandjean et al.
The Warwick study investigated temperature gradients across the cell surface (in line with the stack) and across the cell thickness (perpendicular to the stack) for a wide range of ambient temperatures (-10 ˚C to 50 ˚C)and C rates (0.5C to 10C).
Among their findings were that temperature gradients caused by self-heating increase with increasing C rate and decreasing temperature to such an extent that 13.4 ± 0.7% capacity can be extracted using a 10C discharge compared to a 0.5C discharge, both at −10 °C ambient temperature.
The former condition causes an 18.8 ± 1.1 °C in plane gradient and a 19.7 ± 0.8 °C thermal gradient perpendicular to the stack, which results in large current density distributions and local state of charge differences within the cell.
The top of the cell—close to the tabs—was found not to be the hottest area as commonly represented; metallic bus-bars with high thermal conductivity connected with the cell tabs were identified providing a faster cooling medium.
The gradients result in large current density distributions and local state of charge differences within the cell and battery pack, leading to premature ageing. The temperature gradients measured are much higher than the current industry maximum 5 ˚C thermal gradient across the entire pack used in the automotive industry to minimise localise ageing.—Grandjean et al.
Thomas Grandjean, Anup Barai, Elham Hosseinzadeh, Yue Guo, Andrew McGordon, James Marco (2017) “Large format lithium ion pouch cell full thermal characterisation for improved electric vehicle thermal management,” Journal of Power Sources, Volume 359, Pages 215-225 doi: 10.1016/j.jpowsour.2017.05.016