Thermo-Electrochemical Characterization of LiNi_0.8Co_0.15Al_0.05O₂ /Graphite+Si Full Cells Across Commercial and Academic Form Factors

As electrochemical systems increase in size from academic coin cells to commercial 18650’s and beyond, so too do concerns about how excess heat generation can compromise the cell’s safety and performance. Heat generation in lithium ion battery systems can be attributed to a number of factors, including (1) reversible heat from desired electrochemical reactions (2) joule heating from polarization effects (3) heat of mixing from concertation effects and (4) unwanted side reactions, all of which are influenced by the system’s state of charge (SoC) and operating c-rate. Coupling galvanostatic cycling with isothermal calorimetry enables the observation of heat flow during cell operation and the quantification of the system’s thermodynamic parameters, like enthalpy and entropy. Enthalpy calculations (kJ/mol of Li⁺), found experimentally by the monitoring of heat flow during cell operation, must carefully correlate observed heat signatures and electrochemical processes. This requires low c-rates where overpotentials are suppressed and the observed heat flow is dominated by reversible heat. On the other hand, systematically recording the relationship between temperature and equilibrium voltage at various SoCs provides entropic information and informs the thermodynamics of the system at rest. [1] Theoretically, these thermodynamic measurements should be material specific that rely on the electrochemical redox pair’s SoC, but experimental measurements are also influenced by form factor. When thermo-electrochemical characterization is performed on commercial 18650’s, slow c-rates (&lt;C/10) can be used to ensure that the observed heat is dominated by reversible heat from electrochemical reactions, but thermodynamic conclusions are limited to the cell level because both the cathode’s and anode’s active material exhibit changing entropic behavior with respect to their lithiation state. Academic coin cells, on the other hand, allow for entropic behavior to be attributed to the working electrode’s active material through Li metal half-cell experimentation, but are not as robust as commercial cells.<br/><br/>In this work, entropic and enthalpic experiments were performed on a commercial LiAl_0.8Co_0.15Al_0.05O₂(NCA)/Graphite+Si Li-ion 18650 cell using isothermal calorimetry coupled electrochemical testing. Then, in order to observe contributions from the anode and the cathode separately, the commercial 18650 was disassembled and the recovered electrodes were used to create coin cell half cell with Li metal anodes and coin cell full cells. Coin cell level calorimetry found that the majority of heat generation during (dis)charge originated from the electrode undergoing delithation. Additionally, full cell entropic potential measurements were reconstructed from coin cell half-cell data, elucidating which electrode-specific processes dictate the system’s thermodynamics at different SoCs. Together, heat signature collected across commercial and academic form factors and cell configurations compile an index of thermodynamic and kinetic behaviors of the NCA/Graphite+Si system. These findings will be discussed through a lens meant to inform decisions around commercial cell operation conditions while also serving as an experimental template for thermo-electrochemical comparisons of Li-ion battery systems across commercial and academic form factors.<br/><br/>1. Baek, S.W., et al., Potentiometric entropy and operando calorimetric measurements reveal fast charging mechanisms in PNb₉O₂₅. Journal of Power Sources, 2022. 520: p. 230776.