(Invited) Strategic Cutoff Potential Selection Enables Safe and Long Cycle Life Li4Ti5O12/LiNi0.9Mn0.05Co0.05O2 Batteries for Behind-the-Meter Storage Applications

Abstract

Behind-the-meter storage (BTMS) encompasses storage and distribution systems that bypass the electric grid. For example, energy stored in BTMS stationary batteries can be used for various purposes including electric vehicle charging or lowering the cooling load demands for large buildings. Many previous studies have focused on Li4Ti5O12/LiMn2O4 (LTO/LMO) batteries for BTMS applications1–3 rather than the more conventional electric vehicles comprised of graphite anodes paired with LiFePO4 or layered oxide materials. LTO/LMO batteries are appropriate for BTMS because they provide a favorable balance of cost, safety, and cycle life, which are the most important criteria for BTMS batteries. LTO’s minimal strain, high operating voltage, and relatively earth abundant materials promote cyclability, safety, and reasonable cost.4 LMO is very attractive to pair with LTO for BTMS applications due to abundancy of Mn, safety, and relatively high operating voltage to enable a higher voltage when paired with LTO.5 While LTO/LMO fulfills many requirements for BTMS applications, the high voltage of LTO and the low capacity of LMO limit energy density and specific energy. Improving energy density has the potential to decrease cost in addition to enabling a smaller battery footprint for space-constrained BTMS applications.With the goal of increasing the cathode capacity at high voltage to improve the BTMS battery energy density relative to LTO/LMO, we investigated LTO paired with LiNi0.90Mn0.05Co0.05O2 (NMC90-5-5). We selected NMC90-5-5 for its low Co content to minimize the impact of the cost volatility and supply chain challenges with Co in accordance with BTMS program goals. We investigated long term cycle life and rate capability of LTO/NMC90-5-5 coin cells with two different N/P ratios and two different charge termination potentials. Without the risk of Li plating at high LTO potentials, the cathode can be safely oversized in LTO systems (N/P < 1) such that the excess cathode capacity provides additional Li inventory to overcome losses during cycling and enables longer cycle life. We compared cells fabricated with N/P < 1 with N/P > 1 and we found that cells with N/P < 1 provided higher cell capacity during 1000 cycle tests. We also varied the full cell termination charge potential (2.6 V versus 2.7 V) for each N/P condition. While the 2.7 V termination potential enabled higher capacity, the 2.6 V termination enabled more stable cycling during 1000 cycle tests. Furthermore, because high Ni content layered oxide systems typically exhibit safety challenges,6,7 we also fabricated 18650 LTO/NMC90-5-5 full cells for accelerating rate calorimetry (ARC) testing. ARC testing revealed a significantly lower heating rate as well as a higher temperature where the max heating rate occurred for cells cycled to 2.6 V versus 2.7 V. These data suggest that the termination potential in NMC90-5-5 is critical for optimizing these cathodes for the long cycle life and safety required for BTMS applications.This work was authored in part by the National Renewable Energy Laboratory, operated by Alliance for Sustainable Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. This work was performed, in part, at Sandia National Laboratories, a multi-mission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC (NTESS), a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration (DOE/NNSA) under contract DE-NA0003525. Funding was provided by the U.S. Department of Energy's Vehicle Technologies Office under the Behind-the-Meter Storage (BTMS) Consortium directed by Samuel Gillard and managed by Anthony Burrell. The electrodes used in this manuscript are from Argonne's Cell Analysis, Modeling and Prototyping (CAMP) Facility, which is fully supported by the DOE Vehicle Technologies Office (VTO). The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes.(1) Y. Ha et al., Journal of The Electrochemical Society 2023, 170 (5), 050520.(2) Y. Ha et al., Journal of The Electrochemical Society 2021, 168 (11), 110536.(3) Y. Ha et al., Energy Storage Materials 2021, 38, 581–589.(4) G. Xu et al., Coordination Chemistry Reviews 2017, 343, 139–184.(5) Z. Radzi et al. Journal of Electroanalytical Chemistry 2022, 116623.(6) L. Gan et al., Applied Physics Letters 2022, 121 (20).(7) J. Lamb et al., Journal of The Electrochemical Society 2021, 168 (6), 060516.