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The trade-off between energy density and power capabilities is a challenge for Li-ion battery design as it highly depends on the complex porous structures that holds the liquid electrolyte. Specifically, mass-transport limitations lead to large concentration gradients in the solution-phase and subsequently to crippling overpotentials. The direct study of these solution-phase concentration profiles in Li-ion battery positive electrodes has been elusive, in part because they are shielded by an opaque and paramagnetic matrix. Herein we present a new methodology employing synchrotron hard X-ray fluorescence to observe the concentration gradient formation within Li-ion battery electrodes in operando. This methodology is substantiated with data collected on a model LiFePO 4 /Li cell using a 1 M LiAsF 6 in 1:1 ethylene carbonate/dimethyl carbonate (EC/DMC) electrolyte under galvanostatic and intermittent charge profiles. As such, the technique holds great promise for optimization of new composite electrodes and for numerical model validation.

Lattice oxygen loss during cathode charging significantly limits the charge storage capacity of lithium-ion batteries (LiBs). Therefore, elucidating the oxygen loss and subsequent surface reconstruction phenomena remains an ongoing pursuit with practical implications. We report an in situ oxygen detection strategy using scanning electrochemical microscopy (SECM) that reveals an unprecedented two-stage oxygen evolution behavior from commercial cathodes. This highly sensitive SECM method captured an unreported transient oxygen release at less than 3.3 V vs Li+/Li during the first charge cycle of LiCoO2, LiNi0.33Mn0.33Co0.33O2 and LiNi0.8Mn0.1Co0.1O2. At the main oxygen loss process above 3.3 V vs Li+/Li, SECM mapping highlighted spatial and temporal heterogeneities. Finite element simulations were used to quantify the rate of instantaneous oxygen release, with rates of ~30 pmol/cm2s for the steady-state oxygen evolution. This SECM approach revealed incipient degradation processes and created new quantitative and spatially resolved opportunities for investigating degradation in operating LiB cathodes.

Lithium ion battery performance becomes increasingly limited by ionic transport as the current demand increases. Especially detrimental is the transport within the liquid electrolyte that fills the porous electrode, yet reliable measurement of practical lithium diffusivity within this complex structure has been a longstanding challenge. In this work, we have developed a “single sided” analytical technique to determine the diffusivity in porous networks using scanning electrochemical microscopy (SECM) and a molecular redox marker. SECM surface mapping of porous films shows measurement consistency, and diffusion limited currents through a test structure with well-defined geometry matches the results of numerical modeling within 10%. Diffusivity measurement shows significant deviation from the Bruggeman model for porosities below 60%. The developed technique is applicable to all porous structures independent of their electronic conductivity. Importantly, for lithium-ion batteries the technique does not require free-standing electrodes and therefore is applicable to industrially relevant high power electrodes as a tool for optimization as well as for quality control.

In this work, we provide a theoretical analysis of quantized capacitance (also referred to as solvated Coulomb blockade) as a pseudocapacitive energy storage mechanism. In particular, we examine how redox species exhibiting quantized capacitance might be engineered to satisfy two basic criteria in the design of an “ideal” pseudocapacitive energy storage mechanism: (1) a near-rectangular voltammetric profile which mimics that of double-layer capacitance and (2) a linear rise in the pseudocapacitive current with respect to the voltammetric scan rate. It is demonstrated that nanoparticles exhibiting quantized capacitance may satisfy the first criterion by tailoring their charging and reorganization energies. It is also shown that the second criterion can be satisfied so long as the voltammetric scan rate does not exceed the electron-transfer rate. By formulating a comprehensive theoretical framework for understanding the electron-transfer properties of quantized capacitance, we arrive at a general phenomenological description of how pseudocapacitive properties might be practically engineered through this mechanism.