Scalable bath cell method for reversible lithium management towards near zero volt tolerance in lithium-ion cells
Authors: Crompton Kyle Richard , Hladky Michael , Staub Jason , Landi Brian J. 
Published: February 2020

Improving the safety of lithium-ion batteries has become an increasingly important issue in recent years due to a number of fires and other events including fires aboard airplanes, fires at storage facilities, and personal electronics fires. When in an inactive state, the safety of lithium ion batteries can be improved with methods unique from when the battery is in an active state due the battery not requiring stored energy to perform a task. Namely, completely discharging the cells of an inactive battery to a near zero volt state with an applied resistor can bring a number of safety advantages; 1) no rapid discharge from an internal/external short that can spark thermal runaway, 2) elimination of battery shorting and severe injury risk when assembling large battery packs with voltage >50 V, 3) low-risk, onboard storage of delayed-deployment batteries for aerospace missions, 4) inert battery for insertion of implantable medical devices and 5) enabling formulation of succinct, enforceable and comprehensive regulations for shipping and storing lithium-ion batteries. If lithium-ion batteries can be fabricated to tolerate a near zero volt state without performance loss they can also have improved general overdischarge tolerance, which can lend itself to several advantages beyond safety such as 1) dead-bus recovery, 2) recovery of difficult to access cells such as those in medical implants, 3) asset loss-prevention due to inadvertent overdischarge during storage, and 4) decrease in maintenance requirements for stored cells.

It is well known that a near zero volt state in a conventional lithium-ion cell will lead to severe cell degradation, mainly due to dissolution of the anode copper current collector that results from the high potential (i.e. >3.1 V vs. Li/Li+) that the anode experiences when the cell is in the near zero volt state. Recent developments using reversible lithium management to prevent electrode damage in a near zero volt state have yielded LiCoO2/MCMB and HE5050/MCMB pouch cells with high tolerance to multiple 3 and 7 day near zero volt storage periods. Specifically, the amount of reversible lithium in a cell is modified during cell construction to prevent the anode potential from increasing to higher than the copper dissolution potential (i.e. 3.1 V vs. Li/Li+ at room temperature) and prevent the cathode potential from decreasing to less than the damage potential (1.3 V vs. Li/Li+ for LiCoO2) when the cell is in a near zero volt state. The amount of reversible lithium is iteratively modified based on feedback from reference electrode measurements.

The reversible lithium management approach has several advantages over current commercial approaches in that it does not require secondary active materials, electrode/electrolyte additives, or alternative current collector materials. A reversible lithium management step was also recently integrated into a Solith® semi-automated pouch cell construction line to produce a cell-phone sized pouch cell that maintained nearly 100% of its discharge energy after a 14-day near zero volt storage period under a fixed resistive load. Thus, the reversible lithium management approach is scalable.

A challenge present for the reversible lithium management approach to near zero volt storage tolerance is the detrimental changes of the cathode active material that can result from a low cathode potential (i.e. <1.5 V vs. Li/Li+) in a reversible lithium modified cell in a near zero volt state. Thus, strategies to suppress detrimental changes to the cathode material are important to further improving the reversible lithium management approach to near zero volt storage tolerance. A LiCoO2 cathode was over-inserted with lithium for 10 cycles (Constant current charge to 140 mAh/g, constant current discharge to 3.0 V vs. Li/Li+, fixed resistive load discharge to an additional 7 mAh/g) and found to fade to 96% of its original discharge energy. X-ray diffraction (XRD) data corroborated with previous studies shows that cation exchange in the octahedral layers occurs during the over-insertion of lithium and is responsible for the degraded performance. A solution deposited coating of AlPO4, known to stabilize LiCoO2 to charge potentials >4.2 V vs. Li/Li+, was applied to a LiCoO2 cathode and the same over-insertion cycling was performed. The AlPO2 coated LiCoO2 cathode maintained >99% of its discharge energy after 10 over-insertion cycles, and XRD data showed that the AlPO4 coating decreased the extent of irreversible cation exchange between the octahedral layers in the LiCoO2. Thus, AlPO4 surface coatings can prevent crystal structure changes detrimental to charge/discharge performance in LiCoO2 caused by over-insertion of lithium ions, which can improve the near zero volt storage tolerance of lithium ion cells for which reversible lithium management was applied during cell construction.