Near Zero Volt Tolerant Lithium Ion Cells: Benefits for Safe Storage, Shipping, and Medical Implants

Abstract

There are inherent safety risks associated with lithium ion batteries leading to greater restrictions and regulations on shipping and inactive storage. Maintaining the cell(s) of a lithium ion battery at a near zero voltage with an applied fixed load is one promising approach which can lessen (and potentially eliminate) the risk of entering thermal runaway when in an inactive state. However, a near zero lithium ion cell voltage can be damaging if the anode potential increases to above ~3.1 V vs. Li/Li+ where dissolution of the anode copper current collector and other side reactions occur. Approaches using alternative anode current collectors (e.g. titanium) which are stable at higher electrochemical potentials vs. Li/Li+ have had success in mitigating anode substrate dissolution during storage of cells at near zero voltage. However, several anticipated tradeoffs exist with titanium current collectors including increased foil thickness, increased cost, and lower bulk conductivity compared to standard copper foils. An alternative approach to prevent copper current collector dissolution during near zero volt storage is to ensure that the anode electrochemical potential remains less than the copper dissolution potential during near zero volt storage. Past approaches to achieve this have included secondary electrode active materials with intermediate discharge potentials and high first cycle loss cathode additives. However, addition of secondary active materials or additives to the electrodes that may not contribute to the discharge energy of the cell during normal cycling will reduce cell energy density. Additionally, secondary materials may have other operational concerns such as gassing or instability in the operational potential range of the electrode they are added to. In the present work, the anode potential of a lithium ion cell is maintained less than the copper dissolution potential during near zero volt storage using anode pre-lithiation prior to cell assembly. Specifically, pouch cells were fabricated with a LiCoO2 cathode and a mesocarbon microbead (MCMB) anode that was cycled and partially pre-lithiated with active lithium. Three electrode tests with a lithium metal reference and 1.2 M LiPF6 3:7 EC:EMC electrolyte showed that the electrochemical potential of the anode is less than the copper dissolution potential throughout a multiple day near zero volt storage period. Specifically, during the transient period of cell discharge to near zero volts under fixed load over the course of several hours, the anode potential is <1.9 V vs. Li/Li+. After the cell reaches near zero volts and a quasi-equilibrium state, the electrochemical potential that the electrodes asymptote to, the electrode asymptotic potential (EAP), is ~1.9 V vs. Li/Li+ which is less than the copper dissolution potential. The LiCoO2:MCMB pouch cell maintained 99% of its original capacity and average discharge voltage after three, 3-day storage periods at near zero volts cell voltage under fixed load at room temperature. In a second demonstration, a LiCoO2/MCMB cell with a pre-lithiated anode was stored for 7-day periods under a fixed load at near zero volts. The cell maintained 99% of its original capacity and average discharge voltage after three, 7-day storage periods at near zero volts cell voltage at room temperature. A third LiCoO2/MCMB cell with a pre-lithiated anode maintained >99% of its discharge capacity after a 3-day near zero volt storage period under fixed load at 45 °C. High temperature tolerance could impact the reliability of cells in medical implants, which can be inadvertently overdischarged due to patient non-compliance. Lowering the transient period anode potential and EAP of a cell with anode pre-lithiation is an emerging concept to achieve lithium ion cells with high tolerance to storage at near zero voltage at room temperature and elevated temperatures. The demonstrated cell modifications to achieve this performance utilized conventional materials and required no change of typical cell construction parameters. Thus, anode pre-lithiation is an attractive design for near zero volt storage-tolerant lithium ion cells.