Molecular-level interphase design for sustainable batteries
Since the first commercialization of Li-ion batteries (LIBs) in 1991, they have continued to power the society for decades. However, the ever-growing demands pose challenges to their sustainability in terms of restricted energy density, inadequate cycle life, and limited key resources. In the first part of this thesis (chapter 2), efforts made within Li batteries, with respect to sustainability in energy density, cycle life and resources are reported in three sub-chapters discussed below. Use of pure metallic Li is an important strategy for full utilization of the inherently high energy density of Li. In chapter 2.1, novel research is devoted towards quantifying major Li loss pathways for the first time. Based on the fundamental understanding gained from the quantified correlation between major Li loss forms, a rational interphase design principle for achieving highly reversible lean Li and lean electrolyte Li metal batteries (LMBs) from a holistic perspective is proposed. An inorganic-rich insoluble inner solid-electrolyte interphase (SEI) layer with high electron passivity is established, as well as the suppression of organic SEI dissolution. This work has demonstrated an ultra-low Li loss rate (mainly from Li corrosion and SEI dissolution) of 0.13 μAh cm-2 h-1 and an ultra-low SEI growth rate (mainly from Li corrosion) of 3.20 mΩ cm-2 h-1, leading to over 5000 h Li metal cycling stability at a Li utilization rate of 50%, which is very high in lean Li||Li symmetric cells. Based upon this novel molecular-level interphase design, full LIB cells have been fabricated and validated with promising results. A Li||LiFePO4 (LFP) full cell with lean Li (negative to positive, i.e., N/P ratio of 2) subject to a deep cycling rate of 0.2 C over 700 cycles running over 280 days demonstrates 90% capacity retention at an average Coulombic efficiency (CE) of 99.99%, and impressively a 480 Wh kg-1 Ah-level Li||LiNi0.8Mn0.1Co0.1O2 (NMC811) pouch cell with a lean N/P ratio of 1.02 and a lean electrolyte to capacity (E/C) ratio of 3 g Ah-1 achieves over 90% capacity retention over 160 cycles. In chapter 2.2, novel research with LIBs on using the high energy density of Si, second only to Li, is proposed for mitigating mechanical fracture and loss of electrical contact prevalent with a Si-based anode. A SiOx-based anode, which is capable of alleviating volumetric expansion while strengthening the electrical connectivity in the electrodes with an average CE over 99.9% in a high-loading full cell, is developed based upon a robust Si-O-C covalent bonding by molecular-level interphase wiring. Foundational-level innovative research on recovering the valuable cathodic elements from a spent LIB is devised based upon interphase designs in chapter 2.3. This novel chemically active but mechanically passive photothermal powered device consists of a solar thermal collector interphase with a porous alumina reservoir for Li, which in turn is in interphase with a thin layer of ion-sieving metal organic framework (MOF) separator fed from a solution containing Li+ and Co2+ ions from spent cathodes. A preliminary Life Cycle Assessment (LCA) is reported to validate the potential benefits from such a photothermal recycling strategy in terms of cost, energy consumption, and environmental issues. In the second part of this thesis (chapter 3), research is carried out with respect to Na-ion batteries (NIBs), as a complementary alternative to LIBs deriving benefits with respect to resource sustainability. Na is an attractive lower cost and a more widely available option than Li, and does not depend on the expensive and geographically constrained Co in the cathode. The bill of materials for a NIB is further decreased based upon the replacement of the anodic current collector from Cu to a much cheaper and lighter Al. However, SEI dissolution is much more severe in NIBs than in LIBs, leading to low Na reversibility and poor utilization of Na. The first part in chapter 3 builds a direct correlation between SEI solubility and SEI components, and for the first time quantifies that an organic SEI has 3.26 times the solubility of an inorganic SEI. A novel strategy of preforming an insoluble inorganic-rich SEI has been developed, which contributes to a high-loading hard carbon (HC)||NaMn0.33Fe0.33Ni0.33O2 full cell with 80.0% capacity retention and a record-high 99.95% average CE at 0.33 C over 900 cycles with a commercial electrolyte. In another sustainability validation with NIBs, a novel dual-salt/dual-solvent based electrolyte has been developed that is able to achieve a homogeneous and insoluble SEI. Such molecular-level interphase design contributes to 80.5% capacity retention and a record-high 99.95% average CE at 0.33 C over 1500 cycles in a HC||NaMn0.33Fe0.33Ni0.33O2 full cell. It further leads to an 87.5% state of charge (SOC) capacity in a highly challenging scenario of 4 C fast charging and 0.33 C slow discharging for a high-loading full cell, which demonstrates practical applications for challenging fast charging slow discharging scenarios such as electric vehicles (EVs). In the final part of research with NIBs, an electrolyte design principle based on a low dissolution coefficient, and a new protocol to quantify SEI dissolution have been proposed and developed for the first time. These principles have been validated in a high-loading full cell, achieving a near-unity average CE of 99.98% at 0.33 C over 1000 cycles in a HC||NaMn0.33Fe0.33Ni0.33O2 full cell. This near-unity average CE of 99.98% is not only a record value reported so far for a practical Na-ion full cell, but also satisfies the End-of-Life (EoL) model for the first time in the Na battery field, to the best of my knowledge, providing guidelines for designing stable SEIs with high Na ion reversibility. In summary, this thesis has developed and validated molecular-level interphase design principles that can help pave new ways for enhancing battery sustainability in terms of prolonged cycle life with high energy density and endurable resources.
Battery, Energy density, Interphase, Sustainability, Resource
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